**Summary**
Modifications to fix flash support on the lpc546xx:
- fix IAP entrypoint to be `0x03000204`, not the value at that address
- add a reset and attach sequence before erasing flash sectors. there's
little documentation around this, but experimentally, erasing sector 0
fails when the ROM bootloader is mapped to it (on reset). stepping the
chip once and attaching is enough to snap the chip out of it, permitting
flash erase on sector 0.
- add a few test commands to the lpc546xx table (read_uid, erase/write
sector, etc).
- write the magic CRC value when writing to sector 0
(`lpc_flash_write_magic_vect`).
- move the lpc546xx probe to before the lpc43xx probe, to prevent
getting the lpc546xx into Lockup when reading an illegal memory location
during lpc43xx probing
Fixes#786.
I don't 100% understand the reset/load sequence of this part, but these
changes are sufficient to program and debug the part now.
I didn't do a detailed analysis of what pyocd (via st-link hardware
adapter) and segger jlink do to handle the same, but both of those
worked without modification, so there's some difference in the
sequence they're using.
**Testing**
Verified I can now successfully erase and write an executable in sector
0 (and other sectors).
Verified the new commands work correctly.
