src/sbom/Makefile.inc: Fix variable expansion

Make does its work in two distinct phases. The first one basically
initializes and expands all variables, which are not in a recipe and
the second expands all variables inside recipes and then executes the
recipes if necessary.
Currently on some mainboards it can happen that cpu_microcode_bins
variable is filled with microcode paths AFTER swid-files-y is expanded
in the prerequisite for the sbom rule. That causes the
"$(build-dir)/intel-microcode-%.json pattern matching rule not to be
invoked. At the time, when the recipe is executed however (second phase
of make), swid-files-y will now contain the cpu microcode paths from
cpu_microcode_bins. That causes the goswid tooling to fail since the
necessary files were never created, since
"(build-dir)/intel-microcode-%.json" target was never executed.

In order to trigger the expansion of swid-files-y at the second make
phase (after cpu_microcode_bins is fully filled), this patch makes use
of make's secondary expansion feature.

Before on some boards (including samsung/lumpy) the goswid tool
complained about not finding the microcode sbom files.

Test: build samsung/lumpy with CONFIG_SBOM_MICROCODE=y

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I884469a388fd48be89d74ccda686dd8f299d63eb
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72660
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/sbom/Makefile.inc

@@ -85,7 +85,7 @@ endif
 
 ## Build final SBOM (Software Bill of Materials) file in uswid format
 
-$(build-dir)/sbom.uswid: $(build-dir)/coreboot.json $(swid-files-y) $(swid-files-compiler) | $(build-dir)/goswid $(build-dir)
+$(build-dir)/sbom.uswid: $(build-dir)/coreboot.json $$(swid-files-y) $(swid-files-compiler) | $(build-dir)/goswid $(build-dir)
 	echo "    SBOM      " $^
 	$(build-dir)/goswid convert -o $@ \
 		--parent $(build-dir)/coreboot.json \