zephyr: fix typo in CONFIG_ERASE_PROGRESSIVELY conversion

Kconfig macro was converted with typo to not a MCUboot's
internal macro switch. It was MCBOOT_ERASE_PROGRESSIVELY instead
of MCUBOOT_ERASE_PROGRESSIVELY.
Bug was introduced in
https://github.com/mcu-tools/mcuboot/commit/42c985ceadecb61f77db016ebb64b21099c45f4a



Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>