zephyr: Add MCBOOT_ERASE_PROGRESSIVELY config option

The commit adds generalized MCBOOT_ERASE_PROGRESSIVELY options
that can be used to enable progressive erase of flash in
boot_serial DFU code.
The progressive erase is used to erase flash pages as image
update fragments come, instead of erasing the whole needed
image at the beginning.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>