uclibc: menuconfig: take into account initial settings from config file

When executing the sequence 'make clean uclibc-menuconfig', the configured
config file is not taken into account and one starts from the default
settings.

This patch adds an explicit target for the config file and lets the
configure and menuconfig steps depend on it, fixing the problem.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>