util/abuild: Don't set XGCCPATH if it's in the environment

Change-Id: I0fa231ca3d33300a671810e994c5be54ac10a18b
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/17723
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/abuild/abuild

@@ -31,8 +31,10 @@ export KCONFIG_OVERWRITECONFIG=1
 # path to payload. Should be more generic
 PAYLOAD=/dev/null
 
-# path to coreboot XGCC
-XGCCPATH="${TOP}/util/crossgcc/xgcc/bin/"
+# get path to coreboot XGCC if it's not already set
+if [ -z "$XGCCPATH" ]; then
+	XGCCPATH="${TOP}/util/crossgcc/xgcc/bin/"
+fi
 
 # Add XGCC to the path.
 if [ -d "$XGCCPATH" ] && [[ ":$PATH:" != *":$XGCCPATH:"* ]]; then