Move BR2_ARM_TYPE to an uClibc specific place

BR2_ARM_TYPE is a hidden configuration option that is only used for
the configuration of uClibc, therefore, we move it from
target/Config.arch.in to toolchain/uClibc/Config.in.

We also add a comment that explains that this stuff is only useful for
uClibc <= 0.9.32. Starting from 0.9.33, uClibc build process simply
uses the compiler flags to find the ARM processor that should be
used. So, someday, we'll be able to remove this.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>