doc: cleanup - move board READMEs into respective board directories

Also drop a few files referring to no longer / not yet supported
boards.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Kim Phillips <kim.phillips@freescale.com>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Jason Jin <jason.jin@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Acked-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>