Set and use XGCCPATH

Allow overriding the coreboot toolchain location with `XGCCPATH` so
that xgcc can be installed to a separate location, such as `/opt`.

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

scripts/build.sh

@@ -2,8 +2,8 @@
 
 set -e
 
-# TODO: Move this to build scripts that require it on Debian
-PATH="$PATH:/usr/sbin"
+export XGCCPATH="${XGCCPATH:-$PWD/coreboot/util/crossgcc/xgcc/bin}"
+export PATH="$XGCCPATH:$PATH:/usr/sbin"
 
 if [ -z "$1" ]
 then