mbed: Add CMake support and fix files paths

The mbed port should be compatible with a new command-line tool
mbed-tools (Mbed CLI 2) which use cmake build system. This allows build
cmake-base applications.
Fix mbed files paths - it's required for the new version of mbed-os
(v6.15)

Signed-off-by: Artur Tynecki <artur.tynecki@mobica.com>
diff --git a/boot/mbed/src/secondary_bd.cpp b/boot/mbed/src/secondary_bd.cpp
index 40eb4c8..7550a61 100644
--- a/boot/mbed/src/secondary_bd.cpp
+++ b/boot/mbed/src/secondary_bd.cpp
@@ -21,7 +21,7 @@
 
 #include "flash_map_backend/secondary_bd.h"
 #include "platform/mbed_toolchain.h"
-#include "FlashIAPBlockDevice.h"
+#include "FlashIAP/FlashIAPBlockDevice.h"
 
 /**
  * For an XIP build, the secondary BD is provided by mcuboot by default.