kernel-fitimage.bbclass: introduce get_fit_replacement_type function

kernel-fitimage.bbclass: introduce get_fit_replacement_type function

Avoid to set KERNEL_IMAGETYPE_REPLACEMENT in anonymous python function,
otherwise it chould not be overridden in config files, for instance,
it's being set now in meta/lib/oeqa/selftest/cases/fitimage.py.

Introduce a get_fit_replacement_type function to get the default value
of KERNEL_IMAGETYPE_REPLACEMENT, and it could be overridden in config
files.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>