linker: don't allow libui to SP-HALs and fix warning

libui.so is not used by SP-HALs, so it is removed from the list of libs
exposed from the default namespace.

Also, this fixes a warning message "property value is empty" caused by
the automatically removed trailing '/' for the section 'legacy'. Since
the legacy behavior is already implemented by the linker itself, the
behavior doesn't need to specified in ld.config.txt.

Test: marlin/sailfish boots
Test: no warning message is shown
Change-Id: Ib679794d63b01c6794663dc88f1ab7e72cfb11d3