build!: check boolean flags are not empty

For numeric flags, there is a check for the value to be set. Do the same
for boolean flags. This avoids issues where a flag is defined but
without a value, leading to potential unexpected behaviors.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ib00da2784339471058887e93434d96ccba2aebb2

make_helpers/build_macros.mk

@@ -73,6 +73,7 @@ endef
 # Convenience function for verifying option has a boolean value
 # $(eval $(call assert_boolean,FOO)) will assert FOO is 0 or 1
 define assert_boolean
+    $(if $($(1)),,$(error $(1) must not be empty))
     $(if $(filter-out 0 1,$($1)),$(error $1 must be boolean))
 endef