net: tftp: Move tftp.h file from ./net to ./include/net

This change gives the ability to reuse the <tftp.h> header file by other
subsystems (like e.g. dfu).

Without this change compilation error emerges for the legacy update.c file.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>