[coreboot] Simplify buildrom's handling of coreboot-v2

[Myles Watson]

This patch simplifies supporting v2 boards in buildrom.  I like it pretty
well, but I'm open to ways to simplify it even more.  Mostly what I did was
take the common operations out of the *.mk files and put them in
coreboot.inc.

I'll follow the patch with:

svn rm packages/coreboot-v2/m57sli.mk
svn rm packages/coreboot-v2/tyan-generic.mk
svn rm packages/coreboot-v2/tyan-s2881.mk
svn rm packages/coreboot-v2/tyan-s2891.mk
svn rm packages/coreboot-v2/tyan-s2882.mk
svn rm packages/coreboot-v2/asus_a8v-e_se.mk
svn rm packages/coreboot-v2/qemu.mk
svn rm packages/coreboot-v2/supermicro-h8dmr.mk

They're no longer needed.

Thanks,
Myles

Signed-off-by: Myles Watson