bootloader: create public bootloader_flash.h header

Move non-public functions into bootloader_flash_priv.h header

components/bootloader_support/src/idf/bootloader_sha.c

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 #include "bootloader_sha.h"
-#include "bootloader_flash.h"
+#include "bootloader_flash_priv.h"
 #include <stdbool.h>
 #include <string.h>
 #include <assert.h>