build: don't use $B as the default cwd for functions

build: don't use $B as the default cwd for functions

When bitbake executes a shell or Python function it can cd/chdir() into a
directory before executing the task. If no directory is specified then the
default of $B is used.  However $B is an OpenEmbedded variable and BitBake
shouldn't be aware of it.

To solve this change the semantics slightly so that if no directory is
specified, the current working directory isn't changed.  There's also a sanity
check that emits a warning if a Python task does os.chdir() without restoring
the old path, and the previous working directory is restored.

This does change semantics: whereas before a function in OE would have $B as the
working directory unless specified, now the working directory is the top of the
build tree.  Any breakage this causes can be solved by either adding
do_some_task[dirs] = "${B}" or by using absolute paths in the task.

[ YOCTO #4634 ]

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>