toolchain/crosstool-NG: add C library config munging

For some C libraries, it can be necessary to push the BR settings
down to the libc configuration file. Prepare the infrastructure.

(Note: it will apply only to uClibc for now, and although eglibc
can also be configured, it is not currently supported)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>