utils/pb-udhcpc: Unify references to DHCP bootfile parameter

udhcpc may pass the bootfile parameter as either $bootfile or
$boot_file, depending on whether the option is present in the BOOTP
header, or as a DHCP vendor option. We have code in pb-udhcpc to unify
this to $bootfile, but we only use the unified value in one of the user
events.

This change uses the correct value of bootfile, and fixes the check to
conditionally generate the explicit add event. We also need to update
the user-event code to use the right event parameter name.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>