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>
diff --git a/configs/ds414_defconfig b/configs/ds414_defconfig
index 4125592..8ef2e79 100644
--- a/configs/ds414_defconfig
+++ b/configs/ds414_defconfig
@@ -20,9 +20,8 @@
 CONFIG_DEBUG_UART=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
-CONFIG_BOOTARGS="console=ttyS0,115200"
-CONFIG_USE_PREBOOT=y
-CONFIG_PREBOOT="usb start; sf probe"
+CONFIG_BOOTARGS="console=ttyS0,115200 ip=off initrd=0x8000040,8M root=/dev/md0 rw syno_hw_version=DS414r1 ihd_num=4 netif_num=2 flash_size=8 SataLedSpecial=1 HddHotplug=1"
+# CONFIG_USE_PREBOOT is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SPL_I2C_SUPPORT=y