Add crossgcc target to automatically build reference toolchain

Add crossgcc target to automatically build reference toolchain

This means that a simple:

$ make crossgcc

creates the reference toolchain in the correct directory. Thanks to the
dependency on the clean-for-update target, an existing .xcompile along
with any compiled objects in build/ will be cleaned out, so the next
build will automatically use the newly created reference toolchain.

Signed-off-by: Cristian Magherusan-Stanciu <cristi.magherusan@gmail.com>
Acked-by: Peter Stuge <peter@stuge.se>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@6598 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1