boot: zephyr: Add fallback for overhead calculation when auto fails

Adds a fallback of 128 sectors for calculating the image overhead
size if auto sectors is enabled but cannot calculate the value

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

boot/zephyr/CMakeLists.txt

@@ -600,7 +600,12 @@ if(SYSBUILD)
       if(CONFIG_BOOT_MAX_IMG_SECTORS_AUTO AND DEFINED slot_min_sectors AND "${slot_min_sectors}" GREATER "0")
         math(EXPR boot_status_data_size "${slot_min_sectors} * (3 * ${write_size})")
       else()
-        math(EXPR boot_status_data_size "${CONFIG_BOOT_MAX_IMG_SECTORS} * (3 * ${write_size})")
+        if(CONFIG_BOOT_MAX_IMG_SECTORS)
+          math(EXPR boot_status_data_size "${CONFIG_BOOT_MAX_IMG_SECTORS} * (3 * ${write_size})")
+        else()
+          message(WARNING "CONFIG_BOOT_MAX_IMG_SECTORS is not defined, falling back to 128 sector default. Please set CONFIG_BOOT_MAX_IMG_SECTORS to the required value")
+          math(EXPR boot_status_data_size "128 * (3 * ${write_size})")
+        endif()
       endif()
     else()
       set(boot_status_data_size 0)