improve cross-compile instructions and makefile support

[mflatt]

Add a --cross argument to configure to indicate that the generated makefile's make should be like make kernel, and a pb-based bootstrap should rerun configure to build for the host machine instead of the specified target machine.

Also, add support for DESTDIR as a more conventional alternative to the --temproot configure flag. While we're at it, recognize CFLAGS_FOR_BUILD for building bin/zuo.

As part of the change to configure, having CC_FOR_BUILD set does not by itself disable the use of pb for creating or updating boot files. Depending on how boot files are obtained, --force now may be needed when using CC_FOR_BUILD as part of a cross compile that doesn't use --cross.

Relevant to #847 and #848