Update nuke.c to auto-detect flash size

Fixes #642


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Author: tyeth

flash/nuke/nuke.c

@@ -27,12 +27,11 @@
 
 int main() {
     uint flash_size_bytes;
-#ifndef PICO_FLASH_SIZE_BYTES
-#warning PICO_FLASH_SIZE_BYTES not set, assuming 16M
-    flash_size_bytes = 16 * 1024 * 1024;
-#else
-    flash_size_bytes = PICO_FLASH_SIZE_BYTES;
-#endif
+    uint8_t txbuf[4];
+    uint8_t rxbuf[4];
+    txbuf[0] = 0x9f;
+    flash_do_cmd(txbuf, rxbuf, 4);
+    flash_size_bytes = 1u << rxbuf[3];
     flash_range_erase(0, flash_size_bytes);
     // Leave an eyecatcher pattern in the first page of flash so picotool can
     // more easily check the size: