- added custom bootkernel profile (bnc #846068)
  * with the bootkernel="custom" profile one can prevent the
    installation of a kernel in the kiwi boot image (initrd)
    This makes sense if a non profiled kernel or a specific
    version of a kernel is used as part of the system image
    XML description. Examples:

        <type ... bootkernel="custom"/>

    ==> version pinned kernel:
        <package name="kernel-default-3.7.10-1.16.1" bootinclude="true"/>

    ==> non profiled/custom kernel:
        <package name="kernel-bob" bootinclude="true"/>

    If the custom bootkernel attribute is used it's required to
    mark the kernel in the system image XML with bootinclude="true"
    If this is not done the initrd will not have a kernel/modules
    which is in most cases unwanted