- modify Makefile to enable detect devices directory
- decouple PLIC and UART into separate files
- Bind RISC-V core to plic_t to enable sending interrupt from PLIC to
  core

Currently, the system test suite is compiled into a single ELF
executable, leading to discrepancies between the inputs for Linux
kernel emulation and the system test suite. By introducing
ENABLE_USE_ELF, we can allow the use of the ELF executable when
running the system test suite.

This might be a temporarily solution, need to refactor more.

The MMU test suite relies on rv32emu's syscall function instead of
its syscall table, unlike the Linux kernel. Therefore, use the
ON_TEST macro to distinguish between the Linux kernel and MMU test
suite as the emulation target.

- use stack storage for vm_data_t to eliminate potential mem leak.
- run_and_trace only accessible when USE_ELF is enable since it needs
  parsing the symbol of the ELF. Add TODO on it for parsing Linux kernel
  symbol. In addition, Linux kernel emulation specified code shall be
  checked as well.
- enable parsing different kernel image, rootfs image, and dtb file by
  CLI parameters.
- Linux kernel emulation needs dedicated mem size, but the current
  Makefile default enable SDL if libsdl is installed in system and then
  enable FULL_4G feature. Thus, forcely undefine MEM_SIZE if current
  emulation target is Linux kernel and use the dedicated one.

- remove comment that related to MMU test suite in syscall_handler
  and mention them in Makefile instead
- rename int and ints to intr and intrs respectively
- use enum for PLIC's regs
- Disable termios keyboard echoing and capture CTRL+a+x to exit VM

When you build using the command make ENABLE_SYSTEM=1 -j$(nproc),
the default target in the Makefile is invoked. At this stage, the
Device Tree Blob (DTB) should already be built before executing
rv32emu. Therefore, system.mk is included before the all target
in the Makefile.

- rewrite MMIO handler.
To adapt more MMIO in the future (e.g., VirtIO), the IIF(io) cannot be
used since it supports only binary. Thus, rewrite using switch case can
solve this.

- suppress warning.
In mmu-based read handler, warning 'non-void function does not return a
value in all control paths' could be reached, so add __UNREACHABLE() to
suppress it.

- keyboard hacking should be defined only when Linux kernel emulation.

- opt_trace only available when target RV32 file is ELF executable.

- map_file might be not available on non POSIX compatible machine, thus
  adding a fault back implementation when HAVE_MMAP is not set.

- Stores the default configurations for buildroot, busybox, and Linux
  within the assets directory. Initially, assets were designated for
  wasm usage, but to avoid creating a new directory for configurations,
  the assets directory is reused for this purpose. Also, add an
  automation image build script 'build-img.sh'.
- Add a new Makefile target called 'system' to build and run more easier
  for user.

Since build linux images script is stored in tools/, thus define the
build-linux-img Makefile target in mk/tools.mk for consistency.

With the Linux image and ELF executable already separated, it is
also necessary to decouple the tags. Add the suffix 'Linux-Image'
for the Linux image release artifact and 'ELF' for the test bench
ELF executable.

Additionally, include the Buildroot and Linux kernel version files
in .ci/. Updating either of these files will trigger a rebuild of
the Linux image artifact.

- reuse mk/external.mk to verify integrity of the source.
- let build-system-image Makefile target can be reused in build-artifact
  CI.
- the download, extract and verify function in mk/external.mk are adjusted
  since git CLI command is added to pull buildroot and Linux. Note that
  the '*/.git/*' of a git cloned repository has to be filtered out
  during SHA value verification because the metadata is difference every
  time when cloning.
- all linux image store in build/linux-image directory.

Note that ENABLE_SYSTEM=1 shall be specified

The Linux image rebuild will be triggered if and only if the version of them
are changed in mk/external.mk.

To make PTE variable more semantic. Also, it can be easily to support
more MMU schema afterwards.