Fix exception thrown by boost in MaskToPrefix

Boost Ip4Address() takes unsigned long as argument. If the address
passed is greater than 0xFFFFFFFF, it throws exception. Method
MaskToPrefix has potential to pass values greater than 0xFFFFFFFF.

Renamed MaskToPrefix to PrefixToIpAddress and moved to base/util.h.
The new method uses uint32_t for mask. This ensures the value is
never greater than 0xFFFFFFFF

Change-Id: I64a69c59e7902142e68e09ecef8964a3d79933f7
Closes-Bug: #1390594