Close #69: add 'pmbootstrap initfs' and improve initfs workflow

* allows to build/extract/list initramfs, add/del hook
* rebuild the initfs whenever running install or trying to flash/boot it
* flasher flash/boot: automatically set up a minimal rootfs with kernel and initfs,
  if it does not exist yet

pmb/chroot/initfs_hooks.py

@@ -0,0 +1,67 @@
+"""
+Copyright 2017 Oliver Smith
+
+This file is part of pmbootstrap.
+
+pmbootstrap is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+pmbootstrap is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
+"""
+import os
+import glob
+import logging
+
+import pmb.config
+import pmb.chroot.apk
+
+
+def list_chroot(args, suffix):
+    ret = []
+    prefix = pmb.config.initfs_hook_prefix
+    for pkgname in pmb.chroot.apk.installed(args, suffix):
+        if pkgname.startswith(prefix):
+            ret.append(pkgname[len(prefix):])
+    return ret
+
+
+def list_aports(args):
+    ret = []
+    prefix = pmb.config.initfs_hook_prefix
+    for path in glob.glob(args.aports + "/" + prefix + "*"):
+        ret.append(os.path.basename(path)[len(prefix):])
+    return ret
+
+
+def ls(args, suffix):
+    hooks_chroot = list_chroot(args, suffix)
+    hooks_aports = list_aports(args)
+
+    for hook in hooks_aports:
+        line = "* " + hook
+        if hook in hooks_chroot:
+            line += " (installed)"
+        logging.info(line)
+
+
+def add(args, hook, suffix):
+    if hook not in list_aports(args):
+        raise RuntimeError("Invalid hook name! Run 'pmbootstrap initfs hook_ls'"
+                           " to get a list of all hooks.")
+    prefix = pmb.config.initfs_hook_prefix
+    pmb.chroot.apk.install(args, [prefix + hook], suffix)
+
+
+def delete(args, hook, suffix):
+    if hook not in list_chroot(args, suffix):
+        raise RuntimeError("There is no such hook installed!")
+    prefix = pmb.config.initfs_hook_prefix
+    pmb.chroot.root(args, ["apk", "del", prefix + hook], suffix)