save a last_install file with the result of the last package install attempt

When installing a package, create /cache/recovery/last_install, which
contains the filename of the package and a 1 or 0 for success or
failure.

Also, don't mount ext4 and vfat filesystems as read-only (on devices
where /cache is ext4, we need it to be read-write).

Change-Id: I0cf2a1921bbd65e06343aa74e2006577fac77c2c