build/clean: Add helper script to clean U-Boot builds

Copy the resources/scripts/build/clean/crossgcc script and adapt it to
run "make distclean" on U-Boot build trees. Some build artifacts persist
after the run, so also run "git clean -fdx" if we can.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

resources/scripts/build/clean/u-boot

@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+
+#  helper script: clean the u-boot builds
+#
+#       Copyright (C) 2014, 2015, 2016, 2020 Leah Rowe <info@minifree.org>
+#       Copyright (C) 2015 Klemens Nanni <contact@autoboot.org>
+#       Copyright (C) 2022 Alper Nebi Yasak <alpernebiyasak@gmail.com>
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+# This script assumes that the current working directory is the root
+
+[ "x${DEBUG+set}" = 'xset' ] && set -v
+set -u -e
+
+# clean u-boot builds (dependencies for 'build'):
+
+printf "Cleaning u-boot builds for all boards\n"
+
+[ ! -d "coreboot/" ] && exit 0
+
+# clean u-boot (source archives preserved)
+for board in u-boot/*; do
+    if [ "${board##*/}" = "u-boot" ]; then
+        continue
+    fi
+    if [ ! -d "${board}" ]; then
+        continue
+    fi
+    make -C "${board}/" distclean
+
+    if [ -e "${board}/.git" ]; then
+        git -C "${board}" clean -fdx
+    fi
+done
+
+printf "\n\n"