What luks, lvm and mdadm layouts are bootable?

The only supported by default configuration layout is mdadm at the lowest
layer, then lvm and luks in any order.

(tested on Debian 8, initramfs-tools)

What luks, lvm and mdadm layouts are bootable?

The only supported by default configuration layout is mdadm at the lowest
layer, then lvm and luks in any order.

Multiboot with Grub 2. Do it the hard way ;-)

Upd1. Various small fixes.
Upd2. Change layout.

Table of content:
    A. Notes about grub-install.
    B. Notes about update-grub.
    C. Other notes.
    D. Multiboot schemes.
    E. Scripts and config examples.

initramfs and noexec /tmp

If during creation of new initramfs by update-initramfs (called by e.g.
kernel postinst script) /tmp is mounted with 'noexec', initramfs will be
slightly different and may report errors, like

E: /scripts/local-premount/resume failed with return 255.

Do not mount LV by UUID from fstab.

I should not reference LV in fstab by UUID, because if i have snapshot of this
LV, it is unknown which one (origin or snapshot) will be mounted.