add hardware status HP EliteBook 2570p

See merge request redox-os/redox!1437

Add the build system download and setup instructions on CONTRIBUTING

See merge request !1436

Make QEMU smp and mem options configurable

See merge request !1435

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

See merge request !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 !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 !1431