bitbake.conf, native*.bbclass: pass --sysroot to TOOLCHAIN_OPTIONS

bitbake.conf, native*.bbclass: pass --sysroot to TOOLCHAIN_OPTIONS

Commit 9c5b9db29738a89837917b1a2d8ee5a1cae0b4cc (bitbake.conf: Simplify
TARGET_CPPFLAGS/LDFLAGS) potentially breaks external toolchains, when their
default sysroot is different from our sysroot/staging location.

Pass default --sysroot to compiler/linker through TOOLCHAIN_OPTIONS, avoiding
native and nativesdk packages. This matches the corresponding changes in Poky.
cross.bbclass was already doing it, since it originated from Poky, AFAIK.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Acked-by: Tom Rini <tom_rini@mentor.com>