boot: bootutil: drop flash_area_read_is_empty

Removes the current `flash_area_read_is_empty` which lacked a bit of
clarity in its naming and error handling, as well as requiring an
extra API in the flash map, and switches to using an internal function
`bootutil_buffer_is_erased`.

Code that was previously using `flash_area_read_is_empty` must now be
updated to do a `flash_area_read` followed by a call to
`bootutil_buffer_is_erased` with the read buffer.

The proposal was previously discussed here:

https://github.com/zephyrproject-rtos/zephyr/pull/28519

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
diff --git a/boot/bootutil/src/swap_move.c b/boot/bootutil/src/swap_move.c
index 7332414..8c59843 100644
--- a/boot/bootutil/src/swap_move.c
+++ b/boot/bootutil/src/swap_move.c
@@ -142,12 +142,12 @@
     write_sz = BOOT_WRITE_SZ(state);
     off = boot_status_off(fap);
     for (i = max_entries; i > 0; i--) {
-        rc = flash_area_read_is_empty(fap, off + (i - 1) * write_sz, &status, 1);
+        rc = flash_area_read(fap, off + (i - 1) * write_sz, &status, 1);
         if (rc < 0) {
             return BOOT_EFLASH;
         }
 
-        if (rc == 1) {
+        if (bootutil_buffer_is_erased(fap, &status, 1)) {
             if (rc != last_rc) {
                 erased_sections++;
             }