fix(build): add a default value for INVERTED_MEMMAP

It is needed to check the validity of boolean flags with the updated
macro assert_boolean.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I185beb55606a4ca435d2fee2092fc61725859aa1
diff --git a/Makefile b/Makefile
index 7368ca5..4ef02c6 100644
--- a/Makefile
+++ b/Makefile
@@ -1058,6 +1058,7 @@
 # Variables for use with PRINT_MEMORY_MAP
 PRINT_MEMORY_MAP_PATH		?=	tools/memory
 PRINT_MEMORY_MAP		?=	${PRINT_MEMORY_MAP_PATH}/print_memory_map.py
+INVERTED_MEMMAP			?=	0
 
 # Variables for use with documentation build using Sphinx tool
 DOCS_PATH		?=	docs