Multiarch support

[Eeems]

The reMarkable Paper Pro is aarch64 instead of armv7l. The tool will need to be updated to generate packages for various architectures. Right now we do have the ability to specify multiple architectures, but we use that to define rmall, rm1, and rm2. Our toolchain images are only setup to cross compile for armv7l architectures.

We'll need a better way to handle this, perhaps allowing overriding the image for a specific arch. rmall will also be a concern, as it won't support aarch64, so we'll need to think about splitting out even more architectures. rmall will still work for things that are truly architecture independent, like shell script based packages, but a new split of rm-arm7vl and rm-aarch64 will be required, along with some way to make the toolchain images handle both, preferably automatically. Toltec itself will need to rebuild every package again when this is done to split out the armv7l specific packages and the aarch64 packages.