tools/env: ensure environment starts at erase block boundary

tools/env: ensure environment starts at erase block boundary

56086921 added support for unaligned environments access.
U-boot itself does not support this:
- env_nand.c fails when using an unaligned offset. It produces an
  error in nand_erase_opts{drivers/mtd/nand/nand_util.c}
- in env_sf/env_flash the unused space at the end is preserved, but
  not in the beginning. block alignment is assumed
- env_sata/env_mmc aligns offset/length to the block size of the
  underlying device. data is silently redirected to the beginning of
  a block

There is seems no use case for unaligned environment. If there is
some useful data at the beginning of the the block (e.g. end of u-boot)
that would be very unsafe. If the redundant environments are hosted by
the same erase block then that invalidates the idea of double buffering.
It might be that unaligned access was allowed in the past, and that
people with legacy u-boot are trapped. But at the time of 56086921
it wasn't supported and due to reasons above I guess it was never
introduced.
I prefer to remove that (unused) feature in favor of simplicity

Signed-off-by: Andreas Fenkart <andreas.fenkart@digitalstrom.com>
Acked-by: Stefan Agner <stefan.agner@toradex.com>