vboot: Install files into FW_MAIN_A and FW_MAIN_B unless they're for RO

Setup an initial rule to make use of the updatable CBFS regions in fmap.

Change-Id: I1fe1c6e7574854b735760c85590da6e297f6e687
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/13060
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Makefile.inc

@@ -112,6 +112,7 @@ spc :=
 spc +=
 $(spc) :=
 $(spc) +=
+comma := ,
 
 # files-in-dir-recursive,dir,files
 files-in-dir-recursive=$(filter $(1)%,$(2))