kexec-tools should not exit when there is no /boot/grub

Binary package hint: kexec-tools

When grub is installed but /boot/grub is not configured (e.g. for live CD), kexec-tools (20090000-2.0.0ubuntu11) should not exit when running postinst when installing the kexec-tools deb package. This is the problem I found when using cdebootstrap to create an Ubuntu live system.
How to reproduce it:
1. Install cdebootstrap
2. edit /usr/share/cdebootstrap/suites, append
Suite: karmic
Config: generic-ubuntu
Keyring: ubuntu-archive-keyring.gpg
3. Run:
cdebootstrap --debug --allow-unauthenticated --include=sysv-rc,upstart,system-services,grub,kexec-tools -f minimal karmic ./karmic http://free.nchc.org.tw/ubuntu

It will exit in:
O: Setting up kexec-tools (20090000-2.0.0ubuntu11) ...
P: Configuring package kexec-tools
D: Updating kexec-tools to status 3
O: Generating /etc/default/kexec...
O: Searching for GRUB installation directory ...
O:
O: No GRUB directory found. To create a template run 'mkdir /boot/grub' first. To install grub, install it manually or try the 'grub-install' command. ### Warning, grub-install is used to change your MBR. ###
O:
O: dpkg: error processing kexec-tools (--configure):
O: subprocess installed post-installation script returned error exit status 1

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