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/helpers/frontend.py

@@ -317,6 +317,11 @@ def stats(args):
     pmb.chroot.user(args, ["ccache", "-s"], suffix, log=False)
 
 
+def work_migrate(args):
+    # do nothing (pmb/__init__.py already did the migration)
+    pmb.helpers.logging.disable()
+
+
 def log(args):
     if args.clear_log:
         pmb.helpers.run.user(args, ["truncate", "-s", "0", args.log],