microblaze: Switch to generic bootm implementation

There is no reason to use private code for standard bootm command.
Current implementation is also broken and don't support image relocation
properly. Switching to generic bootm implementation is fixing these issues.

cmdline and bdt bootm subcommands are returning -1 because they are not
implemented.

Similar change was done long time ago by for example commit 2bb5b638
("MIPS: bootm: rework and fix broken bootm code")

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