Do not delete old olsrd configuration file

Upgrading from a old version of olsrd will delete /etc/config/olsrd

This patch will preserve the old configuration file and will also warn the user about the changes in the configuration.

For an easy merge I pulled also @kostko pull request #34 about the missing $IPKG_INSTROOT 


side note: opkg will still try to check the old conffile for modification but will not found it in the ipk archive and so it will complain ( * file_md5sum_alloc: Failed to open file /etc/config/olsrd: No such file or directory. )
I have no clue about how to fix it.

    root@OpenWrt:/tmp# opkg install ./olsrd_0.6.6.2-3_x86.ipk 
    Upgrading olsrd on root from 0.6.3-3 to 0.6.6.2-3...
    Removing obsolete file /etc/config/olsrd.
    Removing obsolete file /etc/init.d/olsrd.
    Not deleting modified conffile /etc/config/olsrd.
    Configuring olsrd.
    **********  WARNING  ***************
    /etc/config/olsrd is no longer used! This will prevent olsrd to start.
    Please use /etc/config/olsrd4 for IPv4 and /etc/config/olrsd6 for IPv6
    Remember to remove option IpVersion from your config file
    Collected errors:
      * file_md5sum_alloc: Failed to open file /etc/config/olsrd: No such file or directory.