microblaze: Fix bug with passing command line

When u-boot passes control over to Linux it places the Linux command
line between to the end of __init_end. When space between
__init_end and __bss_start is not COMMAND_LINE_SIZE then
the part of cmdline can be lost.
In extreme case if __init_end == __bss_start u-boot can't pass
any cmdline to Linux kernel.

This patch fix this issue by copying cmd line directly to
cmd_line char array which is placed in data section.

Reported-by: David Mc Andrew <david.mcandrew@xilinx.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>