microblaze: support U-BOOT image format

Two version are generated.
linux.bin.ub which is created from linux.bin file
and
simpleImage.<dts>.ub which is created from stripped simpleImage.<dts> file

Load address and entry point is for microblaze first instruction
which is CONFIG_KERNEL_BASE_ADDR variable.

There is possible for simpleImage format parse _start symbol too.

simpleImage.<dts> is still stripped elf file

I cleared simpleImage.<dts>.unstrip file because there are so big.

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