rpi-eeprom-digest: Bubble errors from HSM_WRAPPER invocation

[brooswajne]

Noticed when passing a custom HSM wrapper using -H to rpi-eeprom-digest, if my wrapper would fail (exit non-zero) then rpi-eeprom-digest would carry on as if everything was fine and end up with an empty signature being written to boot.sig.

As I understand it, errors in sub-shells ($(...)) inside a command aren't bubbled up by set -e, even if the sub-shell fails the outer echo is the only thing which set -e takes into account. And that call succeeds, so the script reports a success.

Assigning the signature to a variable avoids this shell footgun.

This can easily be tested with a purposefully always-failing wrapper, such as this (which also logs something before exiting):

❯ cat /tmp/badwrapper
#!/bin/sh
echo "wrapper exploded" >&2
exit 1

Before:

❯ ./rpi-eeprom-digest -H /tmp/badwrapper -i /tmp/p.img -o /tmp/p.sig
wrapper exploded
❯ echo $?
0
❯ cat /tmp/p.sig
2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
ts: 1786364891
rsa2048:

After:

❯ ./rpi-eeprom-digest -H /tmp/badwrapper -i /tmp/p.img -o /tmp/p.sig
wrapper exploded
❯ echo $?
1
❯ cat /tmp/p.sig
2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
ts: 1786364960

(the file is still partially written, but there's not much I can do about that without a bigger change - this is still an improvement as at least the overall exit status can be relied on)