[oe] additional bitbake test

[Frans Meulenbroeks]

Hi,

I'd like to propose to add a test in bitbake to see if an overlay dir
exists when specifying overlays.

In the past I bumped into a problem where BBFILE_PATTERN_local =
"^${OE_TOPDIR}/myoverlay/recipes" was not processed properly because
OE_TOPDIR was not passed using BB_ENV_EXTRAWHITE, resulting in the
regexp match with the dir always returning false due to OE_TOPDIR not
being expanded.
A simple test could warn users against errors like this one.

Frans