ath79: lzma-loader: allow setting custom kernel magic

ath79: lzma-loader: allow setting custom kernel magic

...and max flash offset

The mtdsplit parser was recently refactored
to allow the kernel to have custom image header magic.

Let's also do this for the lzma-loader

For example:
When implemented together,
this allows the kernel to "appear" to be a rootfs
by OEM software in order to write an image
that is actually kernel + rootfs.

At the same time,
it would boot to openwrt normally
by setting the same magic in DTS.

Both of the variables
have a default value that is unchanged
when not defined in the makefiles

This has no effect on the size of the loader
when lzma compressed.

Signed-off-by: Michael Pratt <mcpratt@pm.me>