data_smart: Fix remove operator and its interaction with data expansion

If you have:

FOO = "${bindir}/X Y"
FOO_remove = "${bindir}/X"

the expected result is "Y". Currently this doesn't work since the removed
expressions are not expanded first. This patch adjusts things so the
expressions are expanded before being processed for removal.

Also add a test to ensure this case continues to work.

[YOCTO #6624]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
data_smart: Clarify what 'computed' means in the data store history context

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