feat(preset): make boot path configurable through preset variables

Since kernels no longer install directly to /boot, the kernel
destination path needs to be flexible and configurable. This change
adds support for ALL_kerneldest and <preset>_kerneldest variables
in preset files, allowing users to customize where the kernel
should be located for booting.

The implementation exports MKINITCPIO_KERNELDEST during preset
processing, which overrides the hardcoded /boot path when determining
KERNELDESTINATION. This value is then passed to post hooks via the
KERNELDESTINATION environment variable.

Fixes #9

Component: mkinitcpio preset
Signed-off-by: Josephine Pfeiffer <hi@josie.lol>

# Conflicts:
#	man/mkinitcpio.8.adoc
#	mkinitcpio.d/example.preset
#	test/cases/mkinitcpio.bats

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