Split building coreboot toolchains to a separate file

Specify dependencies specifically for building coreboot toolchains in
the file, so they are not conflated with the dependencies required for
building firmware-open.

Remove building the toolchain when building firmware, so that the new
script is the single source for building coreboot toolchains.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

scripts/_build/coreboot.sh

@@ -32,10 +32,6 @@ function check_configs() {
 }
 
 pushd coreboot >/dev/null
-  if [ ! -d util/crossgcc/xgcc ]
-  then
-    make CPUS="$(nproc)" crossgcc-i386
-  fi
   make distclean
   make defconfig KBUILD_DEFCONFIG="${CONFIG}"
   check_configs "${CONFIG}"