uclibc: fixup config before calling menuconfig

The uclibc-menuconfig command is currently run based on the user-specified
config file, while the fixup of the config file is done afterwards.
However, it makes more sense to do an initial fixup _before_ running
menuconfig, so that the options presented to the user are consistent with
the final configuration.

This patch changes the dependency of the menuconfig target from .config to
.stamp_config_fixup_done to achieve this.

Suggested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>