meta: rust: Bug fix for target definitions returning 'NoneType'

meta: rust: Bug fix for target definitions returning 'NoneType'

The build shows below error while building for arm machines.
Exception: TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'
Detailed error info : https://lists.openembedded.org/g/openembedded-core/message/164004

All the target definitions within tspec dictionary are generted as
NoneType. The changes will fix this issue.

Signed-off-by: Sundeep KOKKONDA <sundeep.kokkonda@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>