microblaze: Set script_offset_nor env variable

Set script_offset_nor env variable using CONFIG_BOOT_SCRIPT_OFFSET
and nor flash start address to keep bootscript offset configurable.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>