Replace gummiboot dependency in device-tablet-x64uefi

Upstream-compat check is currently failing, because device-tablet-x64uefi depends on no-longer existing package gummiboot:

It was moved in aports from main to "unmaintained":

fails to build with new gnu-efi, long dead software

So let's replace it with another bootloader. Maybe syslinux?

CC @MartijnBraam, you added the port initially.

FWIW, I built an image with device-tablet-x64uefi recently and ran it on a laptop for alpineconf, worked great :)

We should resolve this timely, so upstream-compat passes again. I should be able to fix it on monday if we don't have a fix already until then.

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