IOM Multiple Init

[oclyke]

The IOM api should allow multiple initialization w/o failure.

Currently this is not the case because the underlying HAL calls will fail if the IOM is already initialized, subsequently failing the MBED_ASSERT.

This was supposed to be caught by checking for a NULL handle, but this check seems inadequate for some reason. Ought to investigate and potentially implement another solution.

iom_init() (should be safe to call multiple times)