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_scratch.c b/boot/bootutil/src/swap_scratch.c
index 99c57f0..55fa61f 100644
--- a/boot/bootutil/src/swap_scratch.c
+++ b/boot/bootutil/src/swap_scratch.c
@@ -110,13 +110,13 @@
     found_idx = 0;
     invalid = 0;
     for (i = 0; i < max_entries; i++) {
-        rc = flash_area_read_is_empty(fap, off + i * BOOT_WRITE_SZ(state),
+        rc = flash_area_read(fap, off + i * BOOT_WRITE_SZ(state),
                 &status, 1);
         if (rc < 0) {
             return BOOT_EFLASH;
         }
 
-        if (rc == 1) {
+        if (bootutil_buffer_is_erased(fap, &status, 1)) {
             if (found && !found_idx) {
                 found_idx = i;
             }