use arbitrary kernel for kdump

[dashesy]

Binary package hint: kexec-tools

Patch for kdump.init.d and kdump.initramfs scripts, new /etc/kdump.conf to allow setting an arbitrary kernel for kdump.
Some kernels (-rt in my case) may not be able to do kdump properly, these patches allow selecting which kernel to use for kdump rather than current kernel.
/etc/init.d/kdump is patched to source the script /etc/kdump.conf. In which KDUMP_KERNEL is set to the kernel release to use in case a crash occurs. The variable kdump_needed also is set to `uname -r` to let the dump kernel know the failing kernel.
initramfs-tools/scripts/init-bottom/0_kdump is also patched to look at kdump_needed value to dump appropriate kernel.
If /etc/kdump.conf does not exist or if it does not set KDUMP_KERNEL previous behavior is achieved (KDUMP_KERNEL=`uname -r`).