fix unnecessarily slow writing of EMMC partitions

These were attempts to write partitions "conservatively" in hopes of
fixing the problems with writing the radio partition on Nexus 4.  They
didn't work (a kernel patch was needed), but got left in.  They make
writing of partitions unnecessarily slow (ie, we really shouldn't need
to sync() after every 4kb).  Roll back most of them, but leave the
verification read-back in.

Change-Id: I94badc0979e88816c5aa0485f6316c02be69173c
Don't abort on read_file if the file is missing.

Change-Id: I85726bf736203d602428114145c3b98692580656
sr: Fix overlay issues

 * Implement double buffering with no memcpy for overlay mode.
 * Move the split display check to the right place, so that the
   sysfs node is actually initialized on initial startup.
 * Cleanup code.

Change-Id: I6d8b249e8ada80fd1a9f83f2e8ca1f762cb78ae5