microblaze: Fix simpleImage format generation

[ Upstream commit ece97f3a5fb50cf5f98886fbc63c9665f2bb199d ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>