bitbake.conf: fix distro-less failure expanding FILESPATH

bitbake.conf: fix distro-less failure expanding FILESPATH

If DISTRO is not set, FILESPATH becomes littered with directories like
files/${DISTRO} etc. It won't bomb until you try to eval it - i.e.
manipulating FILESPATH directly with .= works fine, but calling e.g.
base_set_filespath() throws this:

ERROR: Failure expanding variable FILESPATH, expression was ${@blah} which
triggered exception SyntaxError: EOL while scanning string literal (FILESPATH,
line 1)

(From OE-Core rev: abacd7243a2e4cca216797c4a36ff66ae968ddbd)

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>