Device tree files and overlay files are interconnected
and always located in the same place, separated by
the overlay subdirectory.
${fdtdir}/${fdtfile}
${fdtdir}/overlay/${overlay_prefix}-${overlay_file}.dtbo

By default, we are trying to download a dtb file from
the /boot/dtb folder, which is a link to the real folder.
Depending on the time and type of build, this link may
point directly to the target folder or to a subdirectory,
i.e. possible path options:
/boot/dtb/overlay
/boot/dtb/allwinner/overlay
In the file armbianEnv.txt we can also see:
fdtfile=sun8i-*.dtb
or
fdtfile=allwinner/sun8i-*.dtb

Thus, when the user has frozen the kernel and updated
the BSP package, or vice versa, updated only the kernel,
the following options are possible in the paths after
their concatenation in the script:
/boot/dtb/sun8i-*.dtb
/boot/dtb/allwinner/sun8i-*.dtb
/boot/dtb/allwinner/allwinner/sun8i-*.dtb

Define and remember default path variables.
Provide the user with the ability to specify the path
to the target folder as:
fdtdir=/path/to/dir
Check the existence of the dtb file in all reasonable
paths and set the path to it.
If not found, download the default file from the default path.

Signed-off-by: The-going <[email protected]>