Always rebuild build.prop file

build.prop file was not rebuilt after changes to $(PRODUCT_PROPERTY_OVERRIDES)
Without this patch, if change I681fd92e will be merged,
the wrong date will be always in the zip file's name
(date of building build.prop and not the current date).
This patch simply forces rebuilding build.prop every time.

Change-Id: I5071072cdbb34865e32403a2e7ce0f705ff9daee
Merge git://github.com/CarbonDev/android_build into jb3

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