Fix k64f stack overflow during WebUSB flashing
Fix k64f stack overflow during WebUSB flashing

When flashing via the WebUSB interface (CMSIS v2) the K64F hangs and
USB eventually times out. This is because the main stack overflows
during this operation casuing RTX to trap and loop forever in
osRtxErrorNotify.

This patch prevents a stack overflow by by increasing the main stack
size from 800 bytes to 864 bytes. Note - maximum stack usage recorded
during WebUSB flashing is 816 bytes.

google/DAPLink-port,google/DAPLink-port,google/DAPLink-port,google/DAPLink-port