env: sf: add support of command env erase

Add support of opts erase for env in SPI flash;
this opts is used by command 'env erase'.

This command only fills the env offset by 0x0 (bit flip to 0) and
the saved environment becomes invalid (with bad CRC).

It doesn't erase the sector here to avoid issue when the sector
is larger than the env (i.e. embedded when
CONFIG_ENV_SECT_SIZE > CONFIG_ENV_SIZE).

The needed sector erase will be managed in the next "env save" command,
using the opt ".save", before to update the environment in SPI flash.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>