Add envkernel.sh: instant mainlining environment (#1424)

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

pmb/parse/arguments.py

@@ -240,6 +240,9 @@ def arguments():
     sub.add_parser("shutdown", help="umount, unregister binfmt")
     sub.add_parser("index", help="re-index all repositories with custom built"
                    " packages (do this after manually removing package files)")
+    sub.add_parser("work_migrate", help="run this before using pmbootstrap"
+                                        " non-interactively to migrate the"
+                                        " work folder version on demand")
     arguments_export(sub)
     arguments_flasher(sub)
     arguments_initfs(sub)