loader: simplify efi_cons_poll()

[tsoome]

Summary

EFI_SIMPLE_TEXT_INPUT_PROTOCOL.ReadKeyStroke() does return
with EFI_NOT_READY error if there is no pending keystroke.

And the same is true with
EFI_SIMPLE_TEXT_INPUT_PROTOCOL.ReadKeyStrokeEx().

12.2.3 EFI_SIMPLE_TEXT_INPUT_PROTOCOL.ReadKeyStrokeEx()
UEFI_Spec_Final_2.11.pdf, page 405
12.3.3 EFI_SIMPLE_TEXT_INPUT_PROTOCOL.ReadKeyStroke()
UEFI_Spec_Final_2.11.pdf, page 411

This means, we can use just those functions in efi_cons_poll(),
and if there was key stroke, record it in our local key buffer.

[imp]

So does this work with our arm64 boards that use a u-boot provied EFI image? The comment are for this case, though we really only care about the last u-boot or two, not ancient history.
And will this eat CPU? We've had issues in the past where the boot loader would end up in a tight loop. What assurances are that that this won't happen?

[manu]

In D52494#1198878, @imp wrote:

So does this work with our arm64 boards that use a u-boot provied EFI image? The comment are for this case, though we really only care about the last u-boot or two, not ancient history.

Should be ok, u-boot seems to have waitforkey for quite some times now https://github.com/u-boot/u-boot/blob/master/lib/efi_loader/efi_console.c#L1345

And will this eat CPU? We've had issues in the past where the boot loader would end up in a tight loop. What assurances are that that this won't happen?

[tsoome]

In D52494#1198878, @imp wrote:

So does this work with our arm64 boards that use a u-boot provied EFI image? The comment are for this case, though we really only care about the last u-boot or two, not ancient history.
And will this eat CPU? We've had issues in the past where the boot loader would end up in a tight loop. What assurances are that that this won't happen?

hm, I do not think its a problem with uboot because we are not using WaitForKey(), I still need to check about CPU burn; This change has a bit deeper background and *if* at all, should probably land with followup, but I wanted to collect other opinions and reminders first - the CPU burn issue is something I did forgot about.