ipq806x: add ramdisk feature

This commits adds the "ramdisk" feature to ipq806x target. The
main driving force behind this decision is to facilitate the
installation of OpenWrt on some locked IPQ806x devices.

Examples:

- NEC Aterm WG2600HP

  The U-Boot on WG2600HP is protected with a password which prevents
  users from gaining access to the u-boot prompt in order to install
  the images from there.

  Therefore, on this device, installing OpenWrt by the user involves
  changing the bootcmd as follows so that WG2600HP downloads and
  executes initramfs image from TFTP server.

  ex.:
    bootcmd="ping ${serverip} && tftpboot 0x44000000 wg2600hp-initramfs.bin; bootipq"

- Buffalo WXR-2533DHP

  The U-Boot on WXR-2533DHP has built-in firmware recovery mode.
  It's activated by holding the "AOSS" button during boot. This
  will trigger the device to download the firmware from an TFTP
  server and booting from it. By using this, the user  can the
  install OpenWrt firmware without having access to the UART
  console.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [reworded commit]

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