Reduce srcfs / bare-metal image size to no more than 256MiB

For the Trusted Flash drive project, I have been researching various ways to build some kind of Flash storage (USB, SPI, etc.) that can be verifiably limited to 512 byte capacity at boot, and then fully unlocked later once correct loading of the boot sector (builder-hex0 stage 1) has been verified - all without having to trust some firmware to do the limiting and unlocking.

Some ways to achieve this would be:
* using parallel storage with true address lines, and physically disconnecting/grounding high address lines using a mechanical switch,
* hotplugging SPI flash memory chips (512 byte SPI flash chips do exist, with an interface that's compatible with their larger capacity brethren).

Unfortunately, these techniques all seem to top out at about 256MiB - e.g. 256MiB SPI flash storage is available today, but not 512MiB.

The simplify branch has already achieved a reduction to about 290MiB, so 256MiB seems within reach. Maintaining that goal together with #345 seems rather more difficult, but IMO still doable.