Determine size of bootloader dynamically

[svenrademakers]

Add the necessary sunxi_boot_size function to determine it dynamically from headers:

1. Accept a <F: Read + Seek> (as does erofs_size).
2. Seek to current position + 0x8000. The reason for "current position" (not "start") is so that the function works on either an open u-boot-sunxi-with-spl.bin (good for testing) OR the raw mmcblk0 (which is already seeked to the correct offset).
3. At this new offset will be the contents of u-boot.img, which will either be:

• A legacy/classic U-Boot uImage: header starts with the 4 bytes 27 05 19 56, and the total uImage size is the 64 byte header + body size stored as a big endian u32 at offset 0xC within the header.
• A FIT: header starts with D0 0D FE ED , total header size is a big-endian u32 at offset 0x4, whole header to be parsed with fdt crate (a FIT's header is in standard FDT format). Body size is determined by iterating every child of "/images", summing together the "data-size" and "data-offset" properties of each (as usizes), then selecting the maximum. Total FIT size is header size + body size.

4. Seek back to the original position that the caller expects.
5. Return the boot image size as the u-boot.img size + 0x8000.

Originally posted by @CFSworks in #1 (comment)