uboot-mediatek: several fixes for MT7622

uboot-mediatek: several fixes for MT7622

 * ! Behavior change !

   Old behavior: If the Reset button is held down during power-on
   do factory reset and try booting recovery from TFTP forever.
   If valid recovery is received via TFTP, write it to flash and boot.

   New behavior: If the Reset button is held down during power-on
   do factory reset, then try TFTP *once*, then try booting on-flash
   recovery, then keep trying via TFTP forever until a valid image is
   received.
   Only if there is no bootable recovery stored on flash, store the
   downloaded recovery. Then boot it.

 * Set loadaddr to 0x48000000 in order to allow booting images larger
   than ~47MB (reported by Oskari Lemmelä). Setting loadaddr to
   0x48000000 gives us 384MB on devices with 512MB RAM, which should be
   more than enough as a maximum size for uImage.FIT to be loaded.

 * Widely unify device-specific default environment in preparation to
   auto-generate it from parameters.

 * backport upstream commit fixing MBR/DOS partitioning

Signed-off-by: Daniel Golle <daniel@makrotopia.org>