Fix TFTP bootfile parameter

There was a typo in exec_installer preventing the TFTP bootfile
parameter (DHCP option 67) from being recognized.  As such the
following DHCP configuration would not behave as expected:

  next-server   10.0.1.100;
  bootfile-name name-of-installer-file;

ONIE would just ignore the bootfile during the discovery process.

Testing Done:

Configured a dnsmasq server to send DHCP options 66 (tftp server) and
67 (bootfile) while ONIE ran in installer mode.  ONIE correctly used
the options to download an installer via TFTP.

Reviewed By: vidya sagar ravipati <vidya@cumulusnetworks.com>