opkg: use default PKG_BUILD_DIR

opkg doesn't have BUILD_VARIANTs anymore, so the previously defined
PKG_BUILD_DIR would lead to a weird 'opkg-' path component.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

package/system/opkg/Makefile

@@ -18,7 +18,6 @@ PKG_SOURCE_URL:=http://git.yoctoproject.org/git/opkg
 PKG_SOURCE_DATE:=2011-04-08
 PKG_SOURCE_VERSION:=9c97d5ecd795709c8584e972bfdf3aee3a5b846d
 PKG_MIRROR_HASH:=55e05270f3eb2f3aff5d3791463ce3d13b8197ca7b301cd58e731a249552c48f
-PKG_BUILD_DIR=$(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-$(PKG_VERSION)
 PKG_FIXUP:=autoreconf
 PKG_REMOVE_FILES = autogen.sh aclocal.m4