fastboot: sparse: implement reserve()

In order to process the CHUNK_TYPE_DONT_CARE properly, there is
a requirement to be able to 'reserve' a specified number of blocks
in the storage media. Because of the special handling of "bad blocks"
in NAND devices, this is implemented in a storage abstraction function.

Signed-off-by: Steve Rae <srae@broadcom.com>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>