board: xilinx: Add support for user configurable boot script offset

board: xilinx: Add support for user configurable boot script offset

Currently "script_offset_f" env variable is hardcoded, this variable
specifies from which offset of the flash boot.scr should be read/write.
As flashes are of different sizes having a fixed offset makes it
difficult to load other images into the flash which may overwrite the
boot script or cannot utilize the full memory. This current fix
creates a new config "CONFIG_BOOT_SCRIPT_OFFSET" which holds the
offset address, overwrites the "script_offset_f" variable.
Also removed existing variable with default values, as the default
values are held by CONFIG_BOOT_SCRIPT_OFFSET

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