zephyr: Make boot_serial_extension.c compilation conditional

The commit modifies CMakeLists.txt to include the file into compilation
only when CONFIG_ENABLE_MGMT_PERUSER is selected.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

boot/zephyr/CMakeLists.txt

@@ -92,9 +92,14 @@ zephyr_library_sources(
   flash_map_extended.c
   os.c
   keys.c
-  boot_serial_extensions.c
   )
 
+if(DEFINED CONFIG_ENABLE_MGMT_PERUSER)
+  zephyr_library_sources(
+    boot_serial_extensions.c
+    )
+endif()
+
 if(NOT DEFINED CONFIG_FLASH_PAGE_LAYOUT)
   zephyr_library_sources(
     flash_map_legacy.c