Fix sha256sum on busybox
Busybox's version of `sha256sum` doesn't support `--check` parameter
```
/usr/bin/sha256sum: unrecognized option: check
BusyBox v1.24.2 (2016-08-12 14:38:34 GMT) multi-call binary.
Usage: sha256sum [-c[sw]] [FILE]...
Print or check SHA256 checksums
-c Check sums against list in FILEs
-s Don't output anything, status code shows success
-w Warn about improperly formatted checksum lines
The command '/bin/sh -c ./zcutil/fetch-params.sh' returned a non-zero code: 1```