toolchain/buildroot: fix default of C library choice
The BR2_TOOLCHAIN_UCLIBC symbol doesn't exist, it was meant to be
BR2_TOOLCHAIN_BUILDROOT_UCLIBC.
Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>