package/libbsd: needs an (e)glibc toolchain

libbsd needs support for .init_array and checks for a
glibc >= 2.4 since .init_array was introduced at around
that time.

uClibc claims to be a glibc-compatible toolchain, but it
only impersonates a glibc-2.2.

Just disable libbsd on uClibc.

Fixes:
    http://autobuild.buildroot.net/results/e94/e949d8fabeeecc74bd1c324c516e0b4938c99dbc/
    http://autobuild.buildroot.net/results/d3e/d3e1b70fb91571efacbe32af2cd12d055508f5ac/
    http://autobuild.buildroot.net/results/b19/b19d24dbf9d05d86d839349695da45d548705b25/


    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>