sonic-installer: fix py3 issues in bootloader.aboot

[Staphylo]

These issues are only seen on secureboot enabled platform.

[Staphylo]

I believe we also want this change to be cherry-picked on 202012 if sonic-installer runs in python3 there.

[daall]

@Staphylo there is a merge conflict cherry-picking to 202012 which I guess is probably because we don't have #1366 in 202012. Are those changes needed in the 202012 branch as well?

[Staphylo]

@dall I do believe we need #1366 in 202012 as well.
Let me run some tests on 202012 and I'll confirm this back to you.

I can alternatively make a change against the 202012 branch that coalesce both changes.
Let me know what you prefer.

[Staphylo]

@daall I can confirm that both changes have to be backported.
Let me know if you need my help with anything.

[daall]

Thanks for confirming @Staphylo, I will go ahead and cherry-pick both PRs to 202012! I'll let you know if I run into any problems.