build: allow romcc to be wrapped

Allow ccache and scan-build to wrap romcc.
This works a bit different from the other compilers
because we only define it later.

Change-Id: I3adce91d3dde9dd50aa6a2baad5b457744f35575
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5773
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
2 files changed