kernel-yocto: add KBUILD_OUTPUT to OE_TERMINAL_EXPORTS

kernel-yocto: add KBUILD_OUTPUT to OE_TERMINAL_EXPORTS

Since linux-yocto based recipes have a split build and source directory,
we should export KBUILD=${B} to the devshell. This allows the kernel to
be incrementally build within the shell and not dirty the source
directory (which breaks subsequent full builds).

(From OE-Core rev: 88f88a22dfa730161168b0f228e3954178b74c6a)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>