microblaze: Save and restore first unused vector

Use one memory space to detect little/big endian platforms.
The first unused address(0x28) is used instead 0x0 address (reset vectors).
Detection rewrited reset vector setup from first stage bootloader.

Workflow:
1. Store 0x28 to r7
2. Do little/big endian test
3. Restore r7 to 0x28

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