build-uefi.sh: Clarify the usage instructions

[Baitinq]

This patch adds a bit of clarification on how this script should be used and the needed prerequisites for the script to successfully build coreboot.

P.S. I think this would be useful (at least for me) as it would've saved me quite a lot of time of me banging my head against the wall :))

[MrChromebox]

so, the blobs needed to build any board for which I have a config are already in the 3rdparty/blobs folder, and checking out the blobs submodule is part of the coreboot setup process as documented on doc.coreboot.org

[Baitinq]

I did execute git submodule update --init --checkout but I still needed to include the binary blobs for my thinkpad x230 (maybe they are not present in the included blobs? or I may be very possibly doing something wrong :))

[MrChromebox]

that's correct, since there are lots of variations for the x230's blobs - stock, ME disabled/neutered/shrunk, IFD/BIOS region resized, etc

[Baitinq]

Makes sense, ty. Should I close this pr then? Or maybe change my wording to represent what you've just said/just add the usage comment?