zephyr: Use flash_area_flatten in bs_custom_storage_erase

The intention of bs_custom_storage_erase is to remove data from device;
to support devices that do not require erase, without calling erase,
so that devices that do not implement such functions could work,
the flash_area_erase has been replaced with flash_area_flatten.

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

boot/zephyr/boot_serial_extension_zephyr_basic.c

@@ -47,7 +47,7 @@ static int bs_custom_storage_erase(const struct nmgr_hdr *hdr,
     if (rc < 0) {
         BOOT_LOG_ERR("failed to open flash area");
     } else {
-        rc = flash_area_erase(fa, 0, flash_area_get_size(fa));
+        rc = flash_area_flatten(fa, 0, flash_area_get_size(fa));
         if (rc < 0) {
             BOOT_LOG_ERR("failed to erase flash area");
         }