mainboard/google: Comment variant names in Kconfig

mainboard/google: Comment variant names in Kconfig

It's very confusing trying to find the google platform names, because
they seem all unsorted in Kconfig.  They're actually sorted according
to the variant name, but previously, that was impossible to tell.

- Add a comment to the top of variants in Kconfig.name
- Inset each variant name.  If you start a prompt with whitespace,
it gets ignored, so after trying various ways to indent, the arrow
was the option I thought looked the best.

It now looks like this:
*** Beltino ***

->  Mccloud (Acer Chromebox CXI)

->  Monroe (LG Chromebase 22CV241 & 22CB25S)

->  Panther (ASUS Chromebox CN60)

->  Tricky (Dell Chromebox 3010)

->  Zako (HP Chromebox G1)

Butterfly (HP Pavilion Chromebook 14)

Chell (HP Chromebook 13 G1)

Cheza

*** Cyan ***

Change-Id: I35cb16b040651cd1bd0c4aef98494368ef5ca512
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/26020
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>