env: mmc: fix offsets relative to the end of the partition

env: mmc: fix offsets relative to the end of the partition

According to the help text, you can set negative offsets to indicated
that the offset is relative to the end of the parition. But kconfig
doesn't let you specify negative hex values. I think this fell through
the cracks when converting the symbol from a '#define' to a kconfig
option.

Introduce a new boolean kconfig option to switch on the "relative to the
end" behavior.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>