overlay_add_to_local_fixups: Set fixup_str to value

When looping through the fixup string, the current fixup string should
be set to value on each iteration. This matches the loop in
overlay_fixup_phandle. A future enhancement should "inline"
overlay_add_to_local_fixups to avoid looping through the fixup string
twice.

Fixes commit bdc677a4 ("libfdt: overlay_merge: Ignore unresolved
symbols").

Change-Id: I4d147551c2d9871cfecb8744aaa718ecf28f0bb7
Signed-off-by: Elliot Berman <eberman@codeaurora.org>