Allow changing the kernel partition for fastboot and heimdall in
deviceinfo and on the fly while doing "pmbootstrap flasher
flash_kernel". Also allow changing the partition for
"... flash_rootfs" with fastboot (this was only possible with
heimdall so far).
Introduce two new deviceinfo variables:
* flash_fastboot_partition_kernel
* flash_fastboot_partition_system
This is useful for devices with dual partitioning that have boot_a
and boot_b.