Fix add_soong_config_namespace/add_soong_config_var macros

They were failing with an argument containing whitespace. That is,
calling `$(call add_soong_config_namespace, foo)` would result in
the attempt to assign to an empty variable. It went unnoticed
because `ckati` silently ignores such an assignment (`make` is
more diligent and fails).

Bug: 190051051
Test: treehugger
Change-Id: Ifcc168eaf2db725e705887889e732bcd4410db39