arm: mvebu: board/Synology: Unify legacy kernel support

Move the relevant bits from ds109.{c,h} into common/ and adjust the code
to fit both DS109 and DS414. Moreover:

* Introduce syno_board_id() which translates CONFIG_MACH_TYPE into the
  expected board ID tag value.

* Properly initialize isusbhost, mac and mtu fields from env variables.

* Set the right bootargs/bootcmd to correctly boot legacy kernel out of
  the (DS414) box. Getting the ramdisk location right is a bit tedious.

Cc: Walter Schweizer <swwa@users.sourceforge.net>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Stefan Roese <sr@denx.de>