Don't overwrite boot sector unless OTA changes it

There is a window where the eboot sector is erased and
unwritten/partially written.  If there's a power cycle at this time, the
chip will brick due to eboot being corrupted.

Avoid this by checking if the new eboot 4K sector is identical to the
one already in flash, and if so don't rewrite it.

bootloaders/eboot/eboot.c

@@ -159,11 +159,6 @@ int copy_raw(const uint32_t src_addr,
 	gzip = true;
     }
     while (left > 0) {
-        if (!verify) {
-           if (SPIEraseSector(daddr/buffer_size)) {
-               return 2;
-           }
-        }
         if (!gzip) {
             if (SPIRead(saddr, buffer, buffer_size)) {
                 return 3;
@@ -190,8 +185,25 @@ int copy_raw(const uint32_t src_addr,
                 return 9;
             }
         } else {
-            if (SPIWrite(daddr, buffer, buffer_size)) {
-                return 4;
+            // Special treatment for address 0 (bootloader).  Only erase and
+            // rewrite if the data is different (i.e. very rarely).
+            bool skip = false;
+            if (daddr == 0) {
+                if (SPIRead(daddr, buffer2, buffer_size)) {
+                    return 4;
+                }
+                if (!memcmp(buffer2, buffer, buffer_size)) {
+                    ets_putc('B'); // Note we skipped the bootloader in output
+                    skip = true;   // And skip erase/write
+                }
+            }
+            if (!skip) {
+                if (SPIEraseSector(daddr/buffer_size)) {
+                   return 2;
+                }
+                if (SPIWrite(daddr, buffer, buffer_size)) {
+                    return 4;
+                }
             }
         }
         saddr += buffer_size;