build.sh: change ccache from arch to board

Author: frank-w

build.sh

@@ -102,7 +102,7 @@ if [[ ! $hostarch =~ aarch64|armv ]];then
 	fi
 
 	crosscompile=1
-	export CCACHE_DIR="$HOME/.cache/ccache/${ARCH}"
+	export CCACHE_DIR="$HOME/.cache/ccache/${board}"
 	mkdir -p "${CCACHE_DIR}"
 fi;