Fix TARGET_PATH for external toolchain builds

TARGET_PATH didn't contain $(STAGING_DIR)/usr/bin, which means that
programs installed in $(STAGING_DIR)/usr/bin were not considered for
execution during Buildroot build process. This was a problem with host
automake/autoconf/libtool, which could not be found.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>