support variants of kernels with different cmdlines and/or initrds

[pschichtel]

This PR implements #16.

I'm not entirely happy with the default configuration yet, so consider this a work-in-progress, but it's final enough for you to have a look at it.

[pschichtel]

Also feel free to nitpick about style. I tried to stay consistent with the existing style, but not sure how well that went.

[pschichtel]

Updated the pull request. Also this is the configuration I'm running successfully on my system:

core_cmd="rd.luks.name=___=cryptroot rd.luks.name=___=cryptswap root=/dev/mapper/cryptroot"

SPLASH="/dev/null"
BACKUP=0
KERNEL_VARIANTS=(linux linux-fallback linux-emergency linux-lts linux-lts-fallback)
CMDLINE_DEFAULT="${core_cmd} resume=/dev/mapper/cryptswap rw i915.fastboot=1 quiet loglevel=3 vga=current systemd.show_status=false udev.log_priority=3"

lts_kernel_cmd="${core_cmd} rw"
emerg_kernel_cmd="${lts_kernel_cmd} systemd.unit=emergency.target"

KERNEL[linux-fallback]="linux"
INITRD[linux-fallback]="/boot/initramfs-linux-fallback.img"

KERNEL[linux-emergency]="linux"
INITRD[linux-emergency]="${INITRD[linux-fallback]}"
CMDLINE[linux-emergency]="$emerg_kernel_cmd"

CMDLINE[linux-lts]="$lts_kernel_cmd"

KERNEL[linux-lts-fallback]="linux-lts"
CMDLINE[linux-lts-fallback]="$lts_kernel_cmd"
INITRD[linux-lts-fallback]="/boot/initramfs-linux-lts-fallback.img"

[pschichtel]

18 should probably be merged first and then this one updated.

[pschichtel]

Updated the PR

[andreyv]

Thanks, similar idea implemented in 0ad2b485439c61aa9519b862ff6b6d2738b9207d.