Add Holtek ISP Protocol

[hansemro]

See #12.

Working commands:

• info : prints ISP version, model, page size, flash size
• wipe : mass erase device
• version : get ISP version
• reboot : reboot device

Non-working commands:

• flash : certain regions of flash are left unprogrammed (HT32F1654)
• setversion : impossible to write to ROM

WIP commands:

• dump : dump flash (64 kB always). TODO: get flash size from info.