[nrf fromtree] boot_serial: Fix include

Fixes an include which is needed for multiple options by just
always including it, and fixing the path so it can be included.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 5c5222f)
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

boot/boot_serial/src/boot_serial.c

@@ -67,10 +67,7 @@
 #include "boot_serial/boot_serial.h"
 #include "boot_serial_priv.h"
 #include "mcuboot_config/mcuboot_config.h"
-
-#ifdef MCUBOOT_ERASE_PROGRESSIVELY
-#include "bootutil_priv.h"
-#endif
+#include "../src/bootutil_priv.h"
 
 #ifdef MCUBOOT_ENC_IMAGES
 #include "boot_serial/boot_serial_encryption.h"