buildtools-tarball.bb: Fix PATH for environment setup script

It only added ${SDKPATHNATIVE}/usr/bin to PATH which didn't work when files
were installed to other bin dirs such as /bin or /sbin, for example,
nativesdk-pigz installs the files to /bin, now fix it to keep align with sdk's
PATH.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>