boot: stop appending NUL to .sdmagic and .sbat sections

boot: stop appending NUL to .sdmagic and .sbat sections

Those text sections had a trailing NUL byte. It's debatable whether this is a
good idea or not. Correctly written consumers will look at the section size so
they wouldn't need this. Shim doesn't use a trailing NUL, so let's follow suit.

Fixes https://github.com/systemd/systemd/issues/33731.

898e9edc469f87fdb6018128bac29eef0a5fe698 reworked this code, but didn't actually
change the logic. We have always been appending the trailing zero by using a
NUL-terminated string as the section contents. (I checked this with v253.18
from before the elf2efi rework.)

.sdmagic contains a string like "#### LoaderInfo: systemd-boot 257~devel ####",
which changes with each version, so previous versions would compare unequal
anyway, so we don't need to worry about backwards compatibility.