msm7x30: Enable BOARD_CUSTOM_BOOTIMG flag

* Needed for http://review.cyanogenmod.org/76919

Change-Id: I92222abe84235fa298e41f64bd0c039ffe2d028b
msm7x30: Disable block based OTAs

* Something is broken, probably related to yaffs/mtd
* That prevents zip generation when block mode is used
* Need to revisit this in the future and find possible solutions

Change-Id: I8b97a5716cbde682d04f5c436732e38595aece47