Box 0 can't receive RPC

[Taiki-San]

Multiple mbedOS features don't work properly when uVisor is activated unless implemented in box 0 (Ethernet on a K64F board for instance).

However, exposing those features with a RPC will cause an assert when called because of this code.
The documentation doesn't mention creating RPCs from the public box but it appears possible and when box_id < 1 is patched and replaced with box_id < 0, everything appears to work fine.

Is there a reason for this limitation? If not, would a PR for this patch be accepted?

[ciarmcom]

ARM Internal Ref: IOTSEC-578