sign-tool: Add option to export binary pub key from Signify pub key

[mchack-work]

Description

In order to be able to sign a a vendor pubkey and install it with tkey-mgt -cmd install-pubkey we need to have the binary form of the new key.

If we're using a TKey for signing, we get the pubkey in Signify form from tkey-sign -G. Let's make it possible to convert to binary form with:

sign-tool -P newkey.bin -p newkey.pub

Type of change

Please tick any that are relevant to this PR and remove any that aren't.

• Bugfix (non breaking change which resolve an issue)
• Feature (non breaking change which adds functionality)
• Breaking Change (a change which would cause existing functionality to not work as expected)
• Documentation (a change to documentation)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have added a "Co-authored-by: x" if several people contributed, either pair programming or by squashing commits from different authors.
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)
• QEMU is updated to reflect changes

[dehanj]

The description of -p only reflect when writing to the the file it points two

	pubkeyPath := flag.String("p", "", "File to write pubkey to")

But now it has a usage as both and input and and output path.

[mchack-work]

The description of -p ...
But now it has a usage as both and input and and output path.

Thanks. Fixed.