A fast way to flash incrementally compiled kernels (part II)

Continuation of #1672 (closed), which gotten pretty long and is hard to read now due to some other topics being mixed in.

@drebrez is following this approach (assuming #1685 (closed) is resolved):

$ pmbootstrap build --src=~/Development/kernel-dev/android_kernel_samsung_msm8974/ linux-samsung-klte
$ pmbootstrap flasher flash_kernel

Then we will need someway to build a new package when using the envkernel.sh and the mainline kernel sources

In other words: extend envkernel.sh to run the pmbootstrap build --src line in a convenient way.