functions.mk: allow setting to an empty value

set() is a function of two variables but the and()
check for *both* is incorrect as one might need to
override a previously set variable with empty string;
this has manifested itself with a case like this:

  @$(call set,ROOTPW_EMPTY,1)
  # ...
  @$(call set,ROOTPW_EMPTY,)

lib/functions.mk

@@ -22,7 +22,8 @@ endef
 
 # ...set() comments out any previous definition
 # and then appends an assigning rule...
-set = $(and $(1),$(2),$(set_body))
+# NB: $(2) could be empty
+set = $(and $(1),$(set_body))
 define set_body
 { $(log_body); \
 sed -i 's|^$(1)[ 	]*[+?]*=.*$$|#& # overridden by $@|' "$(CONFIG)"; \