Changes:
* `helpers/envkernel.sh`:
* installs everything needed for kernel compilation in the native
chroot
* mounts the kernel source to `/mnt/linux` inside the chroot
* creates `/mnt/linux/.output` and chowns it to the `pmos` user, that
folder will be used for the kernel build output
* sets up aliases for `make`, `pmbootstrap`, `pmbroot`, `kernelroot`
* new action `pmbootstrap work_migrate`: does the interactive work
folder migration if necessary, otherwise it doesn't output anything
* when calling this first, we can safely use all other commands
non-interactively without showing the output
Benefits:
* Fast setup (especially for people who are new to kernel
compilation
* No need to figure out distribution specific package names
(cross compilers!)
* No need to do a test build just to verify that the right
packages are installed
* Less error prone
* The right dependencies are always installed
* `ARCH` and `CROSS_COMPILE` variables always get set automatically
and based on `deviceinfo_arch`
* If the build environment is broken for some reason, just zap and
start over
* Easy to reproduce problems
Notes:
* `make menuconfig` works as well
* Sourcing was tested with `zsh`, `bash` and `fish`, it should be easy to
extend for other shells
