parse.bootimg: Separate kernel and ramdisk MediaTek headers

Currently, pmbootstrap checks if either the kernel or the ramdisk in a boot.img
contains the MediaTek header, and if one does, it assumes both do. It hardcodes
the label KERNEL for the kernel and ROOTFS for the ramdisk.

My Amazon Echo Dot (gen 2) has a boot.img where only the kernel has the header,
but not the ramdisk. These changes (as well as those in my new boot-deploy MR)
account for that situation (and any possible label an image has) by splitting
bootimg_mtk_mkimage into two variables for the kernel and the ramdisk labels.

Reviewed-by: Oliver Smith <ollieparanoid@postmarketos.org>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/patches/46351