uclibc: Pass ARCH=<TARGET_ARCH>" on make commandline.

uclibc: Pass ARCH=<TARGET_ARCH>" on make commandline.

Like linux kernel Git version of uclibc mandates that ARCH is passed
down from make instead of being computed magically. This will be
there in next release 0.9.31 of uclibc. Defining it on make
invocation for older releases is no harm.

Also move uclibc to use INC_PR

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Henning Heinold <heinold@inf.fu-berlin.de>