Allow building EL and Ubuntu diskless images for a foreign architecture (e.g.
aarch64 on an x86_64 host) by leveraging qemu-user-static. The target
architecture is detected automatically from a -s source tree (for EL),
or may be requested explicitly with the new --arch option.
When the target differs from the host, dnf/debootstrap is invoked with
--forcearch/--arch and the presence of an enabled binfmt_misc handler
with the F (fix-binary) flag is verified up front, so emulation keeps working
inside the installroot chroot and a missing setup yields an actionable
error instead of a confusing exec failure mid-build.
The image architecture is recorded in confluentimg.buildinfo so that
pack selects the initramfs addons for the image architecture rather
than the build host, and exec of a foreign-arch root performs the same
binfmt check.
Markus Hilger
7633bac055