releasetools: Add --oem_no_mount option.

We may have devices with OEM-specific properties but without an OEM
partition (e.g. the properties might be set by init based on hardware
SKUs). For such devices, we supply --oem_no_mount to skip mounting the
OEM partition in the updater-script. The option is only meaningful when
-o (--oem_settings) is specified.

Bug: 27359929
Change-Id: Ic08396e478a82be4188e980e704b33b4f704a8d7
(cherry picked from commit 8608cde9)