We were not correctly passing down TARGET_CPPFLAGS which made external
toolchains not automatically searching for headers in $(STAGING_DIR) to
fail the build. We are also just packaging the library and headers, so
we do not need to build the tests and selftests.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Signed-off-by: Nicolas Thill <nico@openwrt.org>