kobo-librah2o: new device

Basic support for the Kobo Libra H2O ebook reader.

Waveform handling shares the same oddities as for the Kobo Clara HD, the
existing waveform from eMMC is used.

Preferred way to install is to export the eMMC as USB Mass storage,
backing it up, and installing there.

U-Boot is a more recent one than the factory u-boot, supporting easy
access for buttons/LEDs. Pressing PageUp while powering on gives you the
possibility to enable USB mass storage mode to restore your backup.

Kernel is the near mainline kernel also used by the Kobo Clara HD.

It lacks some devicetree additions which are planned for the next
update, but basic things like buttons, usb, serial and wifi are
supported.