Makefile: Revise build options for Linux specific headers

Clean up the feature target by outsourcing the test to an own variable.
Change the print output and don't write to the build-details file.

This is in preparation for further changes.

Change-Id: I18fc27252afb49fa7d1f2787faee2b5b669275aa
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/58627
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

Makefile.d/linux_mtd_test.c

@@ -0,0 +1,8 @@
+#include <mtd/mtd-user.h>
+
+int main(int argc, char **argv)
+{
+	(void)argc;
+	(void)argv;
+	return 0;
+}