[coreboot] [PATCH]Allow relocated .config location

[Patrick Georgi]

Hi,

right now, the build system assumes that .config is in $(top). abuild
stores each board's .config by copying it into the build directory after
the fact, while clobbering any existing files.

With the attached patch, it's possible to define a new location for
.config, using the DOTCONFIG variable, and abuild makes use of that.

It still messes around with .xcompile and the various .config copies, so
this is only the second step of many (I already started on that topic a
while ago, with a minor change to Makefile), but it's a start, and
should be self contained.

==
Allow user to relocate .config by using the DOTCONFIG variable.
Make abuild use that.

Signed-off-by: Patrick Georgi