device-common.mk: Avoid inherit-product-if-exists for inherit-product to avoid silent failures

Not really critial, but it was pointed out that we probably want
explicit failures rather then silent ones if a inherited file goes
missing.

Change-Id: I9358d55ac2f4436acfc28b08a4aa3e7bacd5ab58
Signed-off-by: John Stultz <john.stultz@linaro.org>
1 file changed