BB: uclibc++: make g++-uc* wrappers relocatable

BB: uclibc++: make g++-uc* wrappers relocatable

The g++-uc wrapper hardcodes $(STAGING_DIR) and $(TOOLCHAIN_DIR) paths which
will not work outside of the original build environment.

Replace the hardcoded staging_dir occurences with paths relative to the
$STAGING_DIR environment variable to make the g++-uc* wrappers usable in an
SDK environment.

Fixes the libdb47 build failure reported at
  https://lists.openwrt.org/pipermail/openwrt-devel/2015-April/032455.html

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
Backport of r46162

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@46164 3c298f89-4303-0410-b956-a3cf2f4a3e73