Raspberry Pi PIR motion sensor power supply with Schmitt Trigger

Hello,
I’m trying to develop a power supply for my Raspberry Pi which is being 
switched on by a PIR motion sensor and then the power shall remain on 
until the operating system has shutdown. The power will be supplied by a 
12V battery therefore power consumption of the whole system is 
important. My Raspberry Pi is modified to run with direct 3.3volts like 
described in http://www.daveakerman.com/?page_id=1294
When the motion sensor detects a movement the unit shall start (until 
the boot process the GPIO pin has ~1 volt). After the system starts I’ll 
set GPIO4 to high (3.3 volt). A Schmitt trigger shall supply the power 
until the operating system has shutdown which is being indicated that 
the GPIO4 is set to 0 volts.
From my research in the internet I think that a Schmitt trigger like the 
HCF4093B could be used for the power hysteresis and a switching 
regulator like the LM2596 is the most efficient way to drive the board. 
But I’m not that familiar with electronic designs, so I have some 
questions:
-Would the system work in general like this?
-I have read that the Schmitt trigger requires some capacity. If so how 
can I calculate the capacity which is required and where to connect it?
-As I understood the Schmitt trigger works as comparator. Where do I get 
the voltage to compare with? If I would use here a linear voltage 
regulator it is not efficient anymore!?
I would appreciate your comment and help on this problem.
Axel