uclibc: fix MIPS variables

The BR2_UCLIBC_MIPS_ISA hidden Config.in variable defines which uClibc
config option should be enabled for a given MIPS architecture
variant. Therefore, using lower case names doesn't work: they should
be upper case, to match uClibc config option names. This commit makes
sure the mips32, mips32r2 and mips64 builds select the appropriate
uClibc configuration.

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