Makefile.inc: Fix empty output when processing C struct files in CBFS

Makefile.inc: Fix empty output when processing C struct files in CBFS

When passing $(@) to eval command, $(@) is replaced by empty string,
Also, the $(@) in cbfs-files-processor-struct is a temporary file name,
so we should quote it by an extra '$' or use the arg ($1 or $2)
directly.

For example:
  cbfs-files-processor-struct= \
      $(eval $(2): $(1) $(obj)/build.h $(KCONFIG_AUTOHEADER); \
		# **  $(@) is empty string instead of $(2)   **
		printf "    CC+STRIP   $(@)  \n"; \
		# **  $(1) contains the name of source file  **
		printf "    CC+STRIP   $(1) \n"; \
	......)

Signed-off-by: Xi Chen <xixi.chen@mediatek.com>
Change-Id: Id6a66e25d7dfe8fe6410e517593ed22a438d2f82
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48201
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>