Even in packer 1.5.6, the checksum validation works fine without explicitly
specifing the checksum type, probably using the go-getter length-based
autodetection mentioned in the docs. As such, specifying iso_checksum_type only
prevents building in packer >=1.6.0.
The default alpine MOTD is not really helpful, it may be even confusing,
inviting users to run setup-alpine. Remove it entirely.
I no longer see issues that need resetting terminal mode, removing the related
code.
This does not change much, but provides some more kernel diagnostics to the
serial console, and even though it was not broken, returns getty to the role of
terminal handler (always with passwordless autologin).
