classes/externalsrc: re-run do_configure when configure files change

classes/externalsrc: re-run do_configure when configure files change

If the user modifies files such as CMakeLists.txt in the case of cmake,
we want do_configure to re-run so that those changes can take effect. In
order to accomplish that, have a variable CONFIGURE_FILES which
specifies a list of files that will be put into do_configure's checksum
(either full paths, or just filenames which will be searched for in the
entire source tree). CONFIGURE_FILES then just needs to be set
appropriately depending on what do_configure is doing; for now I've set
this for autotools and cmake which are the most common cases.

Fixes [YOCTO #7617].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>