nixos/crashdump: fix evaluation. (#30415)

The module would fail to evaluate:
```The option value `boot.crashDump.kernelPackages' in ... is not a package.```

Removed the option boot.crashDump.kernelPackage in favor of using
boot.kernelPatches which automatically chooses the same kernel version
as boot.kernelPackage instead of overriding it.

Added option boot.crashDump.reservedMemory to customized crash kernel
memory.

Changed the default of boot.crashDump.kernelParams as the current one
seemed to have no effect.