lib/oe/terminal.py: use an absolute path to execute oe-gnome-terminal-phonehome

lib/oe/terminal.py: use an absolute path to execute oe-gnome-terminal-phonehome

A flaw was found on my Ubuntu 14.04.5 LTS, on which that gnome-terminal is
the default terminal, when I run any of the tasks:
bitbake busybox -c menuconfig/devshell/devpyshell
bitbake virtual/kernel -c menuconfig/devshell/devpyshell

I got a error as follows:
"Failed to execute child process "oe-gnome-terminal-phonehome" (No such file or directory)"

Seems the environment of the process calling Popen is not passed to the
child process, this behaviour is a known issue in Python bug tracker:
http://bugs.python.org/issue8557

It could be fixed by using an absolute path instead per test.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2117c148ef07d84bc605768e3b3671b0126b9337)
Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>