Allow to specify who can set syscfg value

[andrzej-kaczmarek]

It could be useful to restrict where syscfg value can be set. Trying to override value by package which is not allowed should emit an error.

For example:

• @apache-mynewt-core/hw/mcu/nordic/nrf52xxx package defines MCU_NRF52832 and MCU_NRF52840 values which shall be only set by BSP (as it does not make sense to override them elsewhere)
• @apache-mynewt-nimble/nimble/controller package defines BLE_DEVICE which shall not be overriden by other packages since this is used to determine configuration

Proposal:
Add allowoverride attribute to value which can be set to any (default), bsp or none to restrict who can set syscfg value (as explained above). Not sure if other package types (app, target, unittest) make sense here or combinations of these...