Currently pkg/USING says one should use
`EXTERNAL+=$(RIOTBASE)/pkg/<pkg_name>` to enable PKG modules.
Using this line the PKG will be compiled but not linked.
This change adds a USEPKG variable to be used like
`USEPKG += <pkg_name>`, which looks less clumsy and gets the PKG linked
in the binary.
Fixes the following error:
make[1]: Entering directory '/foo/RIOT/pkg/libcoap'
git clone http://git.code.sf.net/p/libcoap/code /foo/RIOT/pkg/libcoap/libcoap && \
cd /foo/RIOT/pkg/libcoap/libcoap && git checkout 89acaa6775ca3a4aedea510557e6b9c0c01fa5db
fatal: destination path '/foo/RIOT/pkg/libcoap/libcoap' already exists and is not an empty directory.
Makefile:25: recipe for target '/foo/RIOT/pkg/libcoap/libcoap' failed
