Allow u-boot repository URL to be overridden

This allows the URL used to clone our specific version of u-boot to be
specified in the environment, instead of being hard-coded.

This new flexibility will be required by the new Continuous Integration
infrastructure, whose test nodes are in a network without direct
internet access.

Change-Id: I7440b5bba6786f979623b4509111e4e99c6558f6

releasetools/fetch_u-boot.sh

@@ -2,6 +2,8 @@
 #
 # Perform a checkout / update the MINIX u-boot git repo if needed
 # 
+: ${UBOOT_REPO_URL=git://git.minix3.org/u-boot}
+
 # -o output dir
 OUTPUT_DIR=""
 GIT_VERSION=""
@@ -38,7 +40,7 @@ fi
 #
 if  [ ! -e "$OUTPUT_DIR" ]
 then
-	git clone git://git.minix3.org/u-boot -b minix $OUTPUT_DIR
+	git clone ${UBOOT_REPO_URL} -b minix $OUTPUT_DIR
 fi
 
 (