uclibc: use $(MAKE) iso $(MAKE1) for menuconfig target

There is no real reason to run uclibc-menuconfig in non-parallel mode,
even though one can neither expect performance benefits from a parallel
menuconfig.
Nevertheless, $(MAKE) is the default, so this patch removes the unnecessary
non-default $(MAKE1) usage for uclibc-menuconfig.

This is a simplification introduced in preparation of the kconfig-package
infrastructure.

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