abuild: rename _builddir to builddir

the _ prefix is reserved for APKBUILD. This is to avoid use variable
names in APKBUILD that may clash with abuilds internal vars

abuild.in

@@ -532,7 +532,8 @@ getpkgver() {
 
 default_prepare() {
 	local i
-	cd "$_builddir"
+	cd "${builddir:-$srcdir/$pkgname-$pkgver}" \
+		|| { error "Is \$builddir set correctly?"; return 1; }
 	for i in $source; do
 		case $i in
 			*.patch)