support lk2nd for htc-m8 and enable initfs_extra

Fix a couple things for htc-m8:

lk2nd support

Like other msm8974 targets, htc-m8 uses lk2nd to boot mainline Linux.

Add support in the lk2nd APKBUILD for a device specific build, since htc-m8 requires non-default values for mkbootimg. Then, put this new lk2nd variant as a dependency on the mainline kernel build. (Note: a fix is required in pmbootstrap to use flasher flash_lk2nd when it's a dependency of a subpackage, see MR pmbootstrap!2511 (merged))

initramfs size issue

Enable initfs_extra, since the initramfs is now too big for htc-m8's boot partition.