Mixed amd64+arm64 USB keys

[zeha]

We introduced arm64 builds.

amd64 host

• arm64 individually works
• mixed install amd64+arm64 works

arm64 host

• arm64 individually works
• mixed install amd64+arm64 works

Alternate outcome for 2024.12:

• arm64 hosts print "unsupported" error message

[zeha]

grml2usb arm64 0.20.3 has multiple problems:

• it still defaults to syslinux
• with --grub, it really wants grub-install to exist
• with --grub, and grub-install available, it complains that syslinux is absent

[zeha]

Mixed install by installing arm64 on arm64 and then amd64 on amd64 was tested and works. Mixed install crossing archs was not tested yet.

[zeha]

Remaining parts moved to 2025.Q1

[zeha]

Moving this forward, as Debian trixie is now frozen.