If SRCPV is set, it can be expanded when SRC_URI doesn't support it
leading to errors. Avoid doing this by setting it only when it makes
sense.
This patch depends on the bitbake python expansion patch series.
[YCOTO #7772]
(From OE-Core rev: ce64da2b80f99c82d96513d8dfb5e572757b0cda)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>