Now we write $SRC/.config.mk.last to store the most recent configure
variables for the backup and restore task. Also, they no longer fail if
they can't find the variables, and will skip the backup/restore stages
and keep going.
Move CFG_TARGET_TRIPLES to CFG_TARGET, since only one target is
supported anyway.
Create config.{mk,tmp,status} files in build directory instead of the
source directory.
This will prevent Makefile errors from affecting backups. This happens when
different branches include Makefile helpers other branches don't have. This
should be more reliable.
