Prefix all build output file names of files which end up in the build

Prefix all build output file names of files which end up in the build
directory with "build/" for consistency (trivial, sort of).

Also, drop printing of "config.g" input file, we usually only print
generated/output files in the build output.

Finally, rename non-existing COMPRESSFLAG variable to
CBFS_PAYLOAD_COMPRESS_FLAG in a printf line. The build output now says

    PAYLOAD    payload.elf l

for payloads (the "l" specifies LZMA compression).

Signed-off-by: Uwe Hermann <uwe@hermann-uwe.de>
Acked-by: Uwe Hermann <uwe@hermann-uwe.de>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@4875 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1