lib: add ram_check_nodie

The current implementation calls die() if memory checking fails.
This isn't always what we want: one might want to print error registers,
or do some other error handling. Introduce ram_check_nodie() for that
reason. It returns 0 if ram check succeeded, otherwise 1.

Change-Id: Ib9a9279120755cf63b5b3ba5e0646492c3c29ac2
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/532
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>