abuild: implement prepare()

prepare is run before build(). this allows uses to do patching before
configuring and building in a separate step.

abuild.in

@@ -291,6 +291,10 @@ runpart() {
 }
 
 # override those in your build script
+prepare() {
+	:
+}
+
 build() {
 	:
 }
@@ -508,7 +512,7 @@ create_apks() {
 apkcache() {
 	if ! apk_up2date || [ -n "$force" ]; then
 		sanitycheck && builddeps && clean && fetch && unpack \
-			&& mkusers && rootpkg || return 1
+			&& prepare && mkusers && rootpkg || return 1
 	fi
 
 	local apk