build system: strip quotes from CONFIG_CBFS_PREFIX in a single location

Instead of having to remember to strip the quotes everywhere so that
string comparisons (of which there are a few) match up, do it right at
the beginning.

Fixes building the image with a .config where CONFIG_CBFS_PREFIX
contains quotes.

Change-Id: I4d63341cd9f0bc5e313883ef7b5ca6486190c124
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/12578
Tested-by: build bot (Jenkins)
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
2 files changed