MFC r252846:

 - Add SRC_FORCE_CHECKOUT configuration option to force svn to checkout
   the src/ tree into a directory that contains files/directories, such
   as a case where a custom kernel configuration file is specified.

 - Allow specification of multiple KERNCONFs to pass to 'make release'.

 - Move evaluation of NODOCS/NOPORTS earlier, and set based on how the
   release process expects these options to be evaluated.

 - Wrap KERNCONF specification in double quotes, and use 'eval' so multiple
   kernel configurations do not cause the build to fail in strange ways.

 - Set WITHOUT_X11 for the port build flags for the documentation toolchain
   build.  Also run 'clean distclean' targets during port build.

PR:		180192
MFC after:	3 days
Approved by:	kib (mentor, implicit)