usb-loader: Get basic USB loader working well

This is good enough to load fernly over USB.
start: Don't relocate to address 0

fernly runs in IRAM now, before PSRAM has been set up.
fernvale.ld: Load at address 0x70000000

We're hacking on PSRAM now, so load to iram.
vectors: Get rid of irq C function

We don't have interrupts yet.
main: Don't enable interrupts

We don't yet have interrupts, so don't enable them.
main: Disable battery watchdog

This allows us to run without a battery.  Otherwise, it would reboot
after about five seconds.