bitbake: toaster: unset BBPATH before starting bbserver for backwards compatibility

bitbake: toaster: unset BBPATH before starting bbserver for backwards compatibility

The latest build enviroment script is now setting BBPATH. This breaks
building previous releases from a master-based Toaster, because with this
newly inherited BBPATH value the 'bitbake.lock' file ends up in the
Toaster build directory instead of the project's build directory.

Toaster should always clear BBPATH so that the bbserver's environment
is clean (enough).

[YOCTO #12363]

(Bitbake rev: 21dde782c049108dd9455ffbf431de214437e800)

Signed-off-by: David Reyna <David.Reyna@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>