kernel: Separate out uboot_prep_kimage

Separate the function which prepares the kernel for packing into
uImage into separate class, so this function can be reused by the
fitImage class.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Koen Kooi <koen@dominion.thruhere.net>
Cc: Paul Eggleton <paul.eggleton@linux.intel.com>
Cc: Ross Burton <ross.burton@intel.com>
Cc: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>