Enable RustPython by default on the "desktop" and "dev" configurations

See merge request redox-os/redox!1434

Use U-Boot with QEMU again if efi=no is used

See merge request redox-os/redox!1433

Also add a nicer error message when the firmware package is not
installed.

Prevent the build cache for the installer getting clobbered

See merge request redox-os/redox!1432

Previously it was getting built by two different toolchains (nightly
from rustup and the relibc prefix) Also some env vars differed between
both invocations causing blake3 and sodiumoxide to get rebuilt.

Use gdb-multiarch for make gdb to support debugging an arm vm on x86 and vice versa

See merge request redox-os/redox!1431

Add pkg-size script

See merge request redox-os/redox!1430

Remove /etc/net/mac

See merge request redox-os/redox!1429

The mac address is now always fetched directly from the network card

Update goals and improve the guide

See merge request redox-os/redox!1428

Add zstd to the bootstrap script

See merge request redox-os/redox!1427