boot: Check current loader file before overwriting

[jlduran]

Summary

Check if the current bootloader file is the same as the new one before
replacing it. If it is, there is no point in doing so.

While here, make sure the copies retain all the attributes of the
original file.

[jlduran]

This is not the right. Sorry.

[jlduran]

My main goal is to achieve idempotence when running install-boot.sh on a running system.
There is an area of ​​the code that I don't understand/I'm afraid of, marked with XXX.

[jlduran]

tools/boot/install-boot.sh

115

I have made a few tests.
If the ESP is small enough to just fit an old boot1.efi, but not the loader.efi, we would remove the only boot loader. Whether such a scenario is realistic is another story.
I'll clean up.