firstboot: Use ':' for no-op command

Previously, shellcheck would warn:

    In eos-firstboot line 9:
    > /var/lib/eos-firstboot
    ^----------------------^ SC2188 (warning): This redirection doesn't have a command. Move to its command (or use 'true' as no-op).

    For more information:
      https://www.shellcheck.net/wiki/SC2188 -- This redirection doesn't have a c...

This is intended to catch cases where the redirect was meant to apply to
the command on the line before.

In this case the usage is correct. Use `:`, which means the same as the
`true` builtin: do nothing, successfully, with no output.

Author: wjt

eos-firstboot

@@ -6,5 +6,5 @@ root_part=$(findmnt -rvnf -o SOURCE /)
 
 resize2fs "${root_part}"
 
-> /var/lib/eos-firstboot
+: > /var/lib/eos-firstboot
 exit 0