buildroot

toolchain-external: musl toolchain on ARM is for MMU-systems only

Now that we have introduced the support for ARM no-MMU in Buildroot,
we need to update the dependencies of the musl external toolchain. It
supports only MMU-capable ARM cores, so it must depend on BR2_USE_MMU,
at least for the ARM platforms.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>