kernel.bbclass: avoid duplicates in KERNEL_IMAGETYPE_FOR_MAKE
Currently if KERNEL_IMAGETYPES contains both vmlinux and vmlinux.gz,
KERNEL_IMAGETYPE_FOR_MAKE will end up containing two copies of
vmlinux, which will result in two calls to "make vmlinux" from
kernel_do_compile().
Avoid duplicating vmlinux in KERNEL_IMAGETYPE_FOR_MAKE plus some
minor non-functional updates to formatting and comments.
Signed-off-by: Andre McCurdy <armccurdy@gmail.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
