fstab: Add fake boot partition

Boot partition is not accessible outside fastboot on semc msm7x30
devices. However we need an fstab entry for /boot in order to get
rid of the device-specific releasetools override.

Change-Id: Ibe6d2d5b515949e786fdf3c55c5f4dab338ac661
Signed-off-by: Michael Bestas <mikeioannina@gmail.com>