populate_sdk_deb: Fix non x86_64 SDK builds

If building with SDKMACHINE=i686 and using the deb backend, populate_sdk
would fail. Its clear when looking at the options that the 32 bit values
were overwritten.

Replace this code with code similar to that used in package_deb itself.

[YOCTO #6458]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>