Recovery-appliable update.zips to (temporarily) kexec boot a kernel. 

boot_zImage.zip:
- Boots an /sdcard/zImage (external SD) kernel.
- Assumes an embedded initramfs, e.g., a stock or stock-based kernel.
- Provides no "bootmode" parameter, i.e., a normal (init.rc) Android boot.

boot_zImage_recovery.zip:
- Boots a /sdcard/zImage (external SD) kernel in "recovery mode".
- Assumes an embedded initramfs, e.g., a stock or stock-based kernel.
- Provides "bootmode=2", i.e., a recovery (recovery.rc) boot.