md5sum: Increase md5sum timeout to 8 seconds/megabyte

This is probably extreme, I think it's only the case if the last block of a compressed written image
is a large number of zeroes, meaning the md5sum has to wait until the last write finishes.

esptool.py

@@ -43,7 +43,7 @@
 CHIP_ERASE_TIMEOUT = 120              # timeout for full chip erase
 MAX_TIMEOUT = CHIP_ERASE_TIMEOUT * 2  # longest any command can run
 SYNC_TIMEOUT = 0.1                    # timeout for syncing with bootloader
-MD5_TIMEOUT_PER_MB = 5                # timeout (per megabyte) for calculating md5sum
+MD5_TIMEOUT_PER_MB = 8                # timeout (per megabyte) for calculating md5sum
 ERASE_REGION_TIMEOUT_PER_MB = 30      # timeout (per megabyte) for erasing a region