boot: Add MCUBOOT_PERUSER_MGMT_GROUP_ENABLED to platforms

The commit adds the MCUBOOT_PERUSER_MGMT_GROUP_ENABLED configuration
option to cypress, mbed and mynewt configurations.
The options is used to enable custom, system specific, mcymgr
commands parsing in mcuboot serial recovery.
For the listed platforms it is set as not enabled (0), as neither
of the platforms currently provide supporting callback.

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