devtool: extract: disable basehash mismatch errors

devtool: extract: disable basehash mismatch errors

Using the setVariable commands here followed by buildFile will result in
"basehash mismatch" errors, and that's expected since we are deviating
*at runtime* from what was previously seen by changing these variable
values. Set BB_HASH_IGNORE_MISMATCH to turn off the errors.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>