Fix an issue in setting up PATH.

`PATH=$(foreach p,$(INTERNAL_USERIMAGES_BINARY_PATHS),$(p):)$$PATH` will
produce a space separated string due to `foreach`, if
$(INTERNAL_USERIMAGES_BINARY_PATHS) has more than one string. We didn't
hit the issue in the past because $(INTERNAL_USERIMAGES_BINARY_PATHS)
had contained only one path in practice.

Test: TreeHugger
Change-Id: I74ef4356668af63d871a81f6bfd4c9324aa1d956