build system: Retire REQUIRES_BLOB

REQUIRES_BLOB assumes that all blob files come from the 3rdparty directory,
builds failed when all files were configured to point to other sources.

This change modifies the blob mechanism so that cbfs-files can be tagged as
"required" with some specification what is missing.

If the configured files can't be found (wrong path, missing file), the build
system returns a list of descriptions, then aborts.

Change-Id: Icc128e3afcee8acf49bff9409b93af7769db3517
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/2418
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martin.roth@se-eng.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Marc Jones <marcj303@gmail.com>

src/Kconfig

@@ -184,16 +184,6 @@ config USE_BLOBS
 	  might be required for some chipsets or boards.
 	  This flag ensures that a "Free" option remains available for users.
 
-config REQUIRES_BLOB
-	bool
-	default n
-	help
-	  This option can be configured by boards that require the blobs
-	  repository for the default configuration. It will make the build
-	  fail if USE_BLOBS is disabled. Users that still desire to do a
-	  coreboot build for such a board can override this manually, but
-	  this option serves as warning that it might fail.
-
 config COVERAGE
 	bool "Code coverage support"
 	depends on COMPILER_GCC