abuild: allow passing custom flags to patch(1) in prepare

To do so you need to set the patch_args variable to your desired
command line flags.

abuild.in

@@ -535,7 +535,7 @@ default_prepare() {
 		case $i in
 			*.patch)
 				msg "$i"
-				patch -p1 -i "$srcdir/$i" || return 1
+				patch "${patch_args:--p1}" -i "$srcdir/$i" || return 1
 				;;
 		esac
 	done