Fix compilation error for install.c in loader

Fix typo in interp_include() invocation (missing 'p')
Remove setting tftpip, as servip is used by the tftp code in libsa. There
is no separate tftpip global variable any more.

Obtained from: Juniper Networks, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25897