Fix microblize little endian toolchain

Buildroot toolchain creates big endian binaries instead of little endian
ones for microblaze architecture. The reason is wrong BR2_ARCH string.
KERNEL_ARCH must contain microblaze in both cases.

Signed-off-by: Jan Drazil <xdrazi00@stud.fit.vutbr.cz>
Signed-off-by: Jan Viktorin <xvikto03@stud.fit.vutbr.cz>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>