Make default HDMI resolution consistent with CONFIG option

Problem:
Even if selecting CONFIG_BOX_FB_1080P, the resolution stays 720p
(reported by A.Sinha).

Solution:
This minor change allows overriding the default HDMI mode setting (720p)
only when the user selects 1080p in the .config, as expected.
Merge pull request #3 from Galland/master

Make default HDMI resolution consistent with CONFIG option