microblaze: Fix simpleImage format generation

simpleImage generation was broken for some time. This patch is fixing
steps how simpleImage.*.ub file is generated. Steps are objdump of
vmlinux and create .ub.
Also make sure that there is striped elf version with .strip suffix.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>