microblaze: Cleanup compiled-in rootfs in BSS section

This patch is based on patch from Steve Magnani.

There were bug for compiled-in rootfs. We have to move
moving rootfs which is in BSS section to _ebss section
which is at the end of kernel and then clear bss section
not vice-versa.

Signed-off-by: Michal Simek <monstr@monstr.eu>