Add script to configure vold based on boot location

On SD card installs, the first partition of the SD card is the FAT
filesystem used as the boot partition.  This leads vold, which searches
the SD card for the first recognized partition when given a partition
number of "auto", to identify /boot as the /sdcard partition.

Fix this by running a script which detects SD card installs and modifies
/fstab.encore to instruct vold to use the last partition on the SD card
in that case.
Configure vold based on boot location in recovery too

To ensure that configure_vold.sh runs before vold starts, we need to use
an init.rc file (postrecoveryboot.sh is run when recovery starts, after
all the various services are started).

We also need to edit the configure_vold.sh script to change the
interpreter location for the recovery (/sbin/sh as opposed to
/system/bin/sh), so we need some build system hackery as well.
Recovery: move fsfinder startup into init.rc

While it doesn't seem to be strictly necessary for fsfinder to be
complete before the recovery starts, it shouldn't hurt either, and it
may prevent some potential problems in cases where recovery has been
instructed to do something when it starts (flash an OTA package, for
example).