Move BR2_SPARC_TYPE to an uClibc specific place

BR2_SPARC_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.

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