installer: add parameter to skip a block device for installation.

A hyper-v user has reported that the ipfire installer detects a wrong drive
as sda and always try to install on this.
Now you can boot the installation with "ipfire skipdst=sda" to install
this may help to install on hyper-v and other situations.
If more than one should skipped add a parameter for every drive.

config/syslinux/syslinux.cfg

@@ -3,6 +3,9 @@ DISPLAY boot.msg
 PROMPT 1
 DEFAULT vmlinuz
   APPEND initrd=instroot vga=791 splash=silent ro
+LABEL ipfire
+  KERNEL vmlinuz
+  APPEND initrd=instroot vga=791 splash=silent ro
 LABEL novga
   KERNEL vmlinuz
   APPEND initrd=instroot ro

src/install+setup/install/mountdest.sh

@@ -1,7 +1,7 @@
 ###############################################################################
 #                                                                             #
 # IPFire.org - A linux based firewall                                         #
-# Copyright (C) 2007-2012  IPFire Team  <info@ipfire.org>                     #
+# Copyright (C) 2007-2013  IPFire Team  <info@ipfire.org>                     #
 #                                                                             #
 # This program is free software: you can redistribute it and/or modify        #
 # it under the terms of the GNU General Public License as published by        #
@@ -89,6 +89,12 @@ for path in /sys/block/*; do
 		fi
 	done
 
+	# Check if user want skip by commandline
+	if [ "$(grep "skipdst=${device_}" /proc/cmdline)" ]; then
+		echo "${device_} was skipped via cmdline."
+		continue
+	fi
+
 	echo "Checking ${device_}"
 	if check_source_drive ${device_}; then
 		echo "  is source drive - skipping"