Embedded specific fixes

[joaohf]

Hi there

These two commits are necessary in order to run relx builtin_hooks and extended_bin scripts when the target platform doesn't provide bash and relies on busybox utilities.

• use sh instead of bash
• call dd when reading /dev/urandom and use common od options (supported by busybox and GNU utils)

Please, feel free to close this PR if it is very embedded specific use case.

Thanks.

[tsloughter]

Oh thanks, I guess I hadn't looked at those hooks in forever.. made a point of linting the extended start script for non-posix shell stuff in the last release so this is great to hopefully complete this process.

[lrascao]

👍