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>
diff --git a/board/xilinx/zynqmp/zynqmp.c b/board/xilinx/zynqmp/zynqmp.c
index b72eade..8bdc677 100644
--- a/board/xilinx/zynqmp/zynqmp.c
+++ b/board/xilinx/zynqmp/zynqmp.c
@@ -667,6 +667,8 @@
 	initrd_hi = round_down(initrd_hi, SZ_16M);
 	env_set_addr("initrd_high", (void *)initrd_hi);
 
+	env_set_hex("script_offset_f", CONFIG_BOOT_SCRIPT_OFFSET);
+
 	reset_reason();
 
 	return 0;