boot/zephyr: make flash_area_erased_val() weak

The function was made week so zephyr-rtos implementation
will be used if available.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index e7b9c1b..7b6ae63 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -110,7 +110,7 @@
 }
 
 #define ERASED_VAL 0xff
-uint8_t flash_area_erased_val(const struct flash_area *fap)
+__weak uint8_t flash_area_erased_val(const struct flash_area *fap)
 {
     (void)fap;
     return ERASED_VAL;