bitbake: bitbake: toaster: allow OE_ROOT to be provided through environment

bitbake: bitbake: toaster: allow OE_ROOT to be provided through environment

Updates for YOCTO #12891 allowed a user to have a directory
structure different to that of yocto (bitbake isn't inside
oe-core) whereas the definition of OE_ROOT in the main toaster
binary still assumes the same while checking for .templateconf
and hence we see an error on the cmdline in such cases:
bash: <repo-path>/bitbake/bin/../../.templateconf: No such file or directory
The change here now allows the user to provide OE_ROOT through
the environment in such cases and otherwise defaults to the older
mechanism to fix this issue.

(Bitbake rev: 9cc6fe4be797e106899d2448797a4b3af8eace4e)

Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>