checkconf.mk: do not use full path to generate guard symbol in conf.h

The combination of building with -g3 (which emits definitions of all
defined preprocessor macros to the debug info) and using a full path
to define the name of this preprocessor guard means that the output is
not binary reproducible across different build hosts. For example, in
my Yocto build, the string

  __home_ravi_yocto_tmp_glibc_work_stm32mp135fdk_oe_linux_gnueabi_optee_os_stm32mp_3_19_0_stm32mp_r1_1_build_stm32mp135f_dk_include_generated_conf_h_

appears in several build artifacts. Another developer or buildbot
would not build in some /home/ravi/... directory.

In order to increase binary reproducibility, only use the path sans
the $(out-dir)/ prefix of the conf.h file.

Reviewed-by: Jens Wiklander <[email protected]>
Signed-off-by: Rasmus Villemoes <[email protected]>

mk/checkconf.mk

@@ -17,7 +17,8 @@ define check-conf-h
 	cnf='$(strip $(foreach var,				\
 		$(call cfg-vars-by-prefix,$1),			\
 		$(call cfg-make-define,$(var))))';		\
-	guard="_`echo $@ | tr -- -/.+ _`_";			\
+	guardpath="$(patsubst $(out-dir)/%,%,$@)"		\
+	guard="_`echo "$${guardpath}" | tr -- -/.+ _`_";	\
 	mkdir -p $(dir $@);					\
 	echo "#ifndef $${guard}" >[email protected];			\
 	echo "#define $${guard}" >>[email protected];			\