arm: make 'MAKEALL -a' distinguish between arm and aarch64

The vexpress_aemv8a is the first aarch64 board in U-Boot.
As it was introduced, it gets built when "MAKEALL -a arm"
is invoked, and fails as this command is run with a 32-bit,
not 64-bit, toolchain as the cross-compiler.

Introduce 'aarch64' as a valid 'MAKEALL -a' argument, treated
as 'arm' for all other intents, and change the architecture
of the vexpress_aemv8a entry in boards.cfg from 'arm' to
'aarch64'.
diff --git a/mkconfig b/mkconfig
index 40db991..b96c81f 100755
--- a/mkconfig
+++ b/mkconfig
@@ -85,6 +85,13 @@
 	exit 1
 fi
 
+#
+# Test above needed aarch64, now we need arm
+#
+if [ "${arch}" = "aarch64" ]; then
+	arch="arm"
+fi
+
 if [ "$options" ] ; then
 	echo "Configuring for ${BOARD_NAME} - Board: ${CONFIG_NAME}, Options: ${options}"
 else