util/abuild: Clean up the missing_arches check

This change adds the following improvements:
* Easier to read.
* Checks to see if .xcompile is complete.
* Checks the make return code. This will catch if .xcompile is missing.

BUG=b:112267918
TEST=Modified my .xcompile and ran abuild and verified that
missing_arches got set correctly. Also deleted .xcompile and verified
there was a failure.

Change-Id: I7604d431f398fc0c80a857a0c7c21e164004cc99
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34241
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

util/abuild/abuild

@@ -462,10 +462,26 @@ function build_config
 		return
 	fi
 
+	local required_arches
+
 	required_arches=$(grep -E "^CONFIG_ARCH_(BOOTBLOCK|R.MSTAGE|VERSTAGE)" "$TARGET/${BUILD_NAME}/config.build" | \
 			sed "s,^CONFIG_ARCH_[^_]*_\([^=]*\)=.*$,\1," |sort -u |tr 'A-Z\n\r' 'a-z  ')
-	# shellcheck disable=SC2016,SC2059
-	missing_arches=$(printf 'include .xcompile\nall: ; @echo $(foreach arch,'"$required_arches"',$(if $(filter $(arch),$(SUBARCH_SUPPORTED)),,$(arch)))' | $MAKE --no-print-directory -f -)
+
+	missing_arches="$($MAKE --no-print-directory -f - \
+		REQUIRED_ARCHES="$required_arches" <<'EOF'
+include .xcompile
+.PHONY: missing_arches
+missing_arches:
+	$(if $(XCOMPILE_COMPLETE),,$(error .xcompile is invalid.))
+	@echo $(foreach arch,$(REQUIRED_ARCHES),$(if $(filter $(arch),$(SUBARCH_SUPPORTED)),,$(arch)))
+EOF
+)"
+	# shellcheck disable=SC2181
+	if [[ $? -ne 0 ]]; then
+		echo "Calculating missing_arches failed" >&2
+		exit 1
+	fi
+
 	if [ -n "$missing_arches" ]; then
 		printf "skipping %s because we're missing compilers for (%s)\n" "$BUILD_NAME" "$missing_arches"
 		return