filemap.py: enforce maximum of 4kb block size

filemap.py: enforce maximum of 4kb block size

The logic in this script validates that the length of data sections are
evenly divisible by the block size. On most systems the block size is
4KB and all is good. Some systems though, such as ppc64le, have a block
size larger then 4KB. For example on a POWER9 based ppc64le system, the
block size is 64KB.

This results in this script failing with errors like this when building
wic images:
|440, in _do_get_mapped_ranges
|     assert extent_len % self.block_size == 0
|            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| AssertionError

In this case the data section size was 268KB and the block size was
64KB, resulting in the above assert failure.

Resolves https://bugzilla.yoctoproject.org/show_bug.cgi?id=15075

Signed-off-by: Andrew Geissler <geissonator@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>