toolchain: do not allow BR2_GENERATE_LOCALE with musl

Generating locales is possible in two situations:

 - With the internal toolchain backend, when the uClibc library is
   used. With uClibc, locales are generated at build time of the C
   library, so with uClibc it's only possible with the internal
   toolchain backend.

 - With either the internal or external toolchain backend when the
   glibc library is used. With glibc, locales can be generated
   afterwards, using the host-localedef utility.

Until we had the musl C library supported in the internal toolchain
backend, the condition: BR2_TOOLCHAIN_BUILDROOT ||
BR2_TOOLCHAIN_USES_GLIBC was correct to capture the above two
situations. Now that we have musl support in the internal toolchain
backend, then BR2_TOOLCHAIN_BUILDROOT is incorrect, and we should use
BR2_TOOLCHAIN_BUILDROOT_UCLIBC instead.

Basic locale support in musl has appeared in musl 1.1.4, but we are
not yet capable of generating the locale files for musl.

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