Add pulp-offload-intf.c to stimulate and test CVA6-PULP cluster inter…

…ference (pulp-platform#262)

* WIP: add wce test.

* Fix `buffer` accesses in `pulp-offload-intf.c`

* Let `pulp_boot_default` be defined by `elf2header.py`

* Point to yt/carfield branch for the `regression_tests`

* Fix `carfield.mk` indentation

* Correctly filter out `CAR_ELFLOAD_PULPD_INTF_SRC_C` from `CAR_SW_TEST_SRCS_C`

sw/sw.mk

@@ -120,8 +120,6 @@ car-pulpd-sw-offload-tests:
 	$(call offload_tests_template,$(PULPD_HEADER_TARGETS),pulpd,$(CAR_ELFLOAD_BLOCKING_PULPD_SRC_C),$(CAR_ELFLOAD_BLOCKING_PULPD_PATH))
 	$(call offload_tests_template,$(PULPD_HEADER_TARGETS),pulpd,$(CAR_ELFLOAD_PULPD_INTF_SRC_C),$(CAR_ELFLOAD_PULPD_INTF_PATH))
 
-endif
-
 # Litmus tests
 LITMUS_REPO := https://github.com/pulp-platform/CHERI-Litmus.git
 LITMUS_DIR  := $(CAR_SW_DIR)/tests/bare-metal/riscv-litmus-tests