Add interposition fixes for use with APK Foundry

• Make $APK consistently used
• Change $git to $ABUILD_GIT and make it consistently used
• Change $srcdir override to $ABUILD_SRCDIR to prevent collisions with shell scripts in packages being built
• Ditto $pkgbasedir and $ABUILD_PKGBASEDIR
• Add $ABUILD_FETCH override for abuild-fetch

abuild-keygen.in

@@ -25,7 +25,7 @@ get_privkey_file() {
 
 	# if PACKAGER does not contain a valid email address, then ask git
 	if [ -z "$emailaddr" ] || [ "${emailaddr##*@}" = "$emailaddr" ]; then
-		emailaddr=$(git config --get user.email 2>/dev/null)
+		emailaddr=$($ABUILD_GIT config --get user.email 2>/dev/null)
 	fi
 
 	default_name="${emailaddr:-$USER}-$(printf "%x" $(date +%s))"