ramips: use different board names for variants

Use different names for flash size related board variants, to make sure
that only images for the actual flash size are considered as valid by
the image validation code.

Remove the flash size suffix from the string returned by
ramips_board_detect() to ensure that existing scripts relying on the
former used boardname are still working.

Signed-off-by: Mathias Kresin <dev@kresin.me>