With the device tree overlays mechanism in place, the bootscript
doesn't need to set the fdt_file variable. Instead, it will use
the value it has (which can be changed by the user) and it will
simply update the 'overlays' variable with the device tree
overlays that apply basing on the hardware capabilities found
on the HWID, SOM version, and carrier board version.
This allows a user to override the default fdt_file to point
to his custom device tree, but still make use of the boot script
for hardware-detected overlays.
Without the need to set the base filename, the boot script is the
same for any carrier board of the ccimx8x SOM, so this commit
moves it to a common folder.
Signed-off-by: Hector Palacios <hector.palacios@digi.com>