uclibc: do not use GENERATE_LOCALE

Since commit 33de7401 ("Makefile:
convert "target-generatelocales" to a hook"), the GENERATE_LOCALE
variable is never defined when using uClibc. This means that setting
any value to the BR2_GENERATE_LOCALE option had no effect for uClibc
toolchains.

In this commit, we make sure the uclibc.mk logic re-creates its own
qstripped version of BR2_GENERATE_LOCALE, in a variable called
UCLIBC_GENERATE_LOCALES.

This fixes locale generation with uClibc in the internal toolchain
backend.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>