is the enable high or low?
assuming enable is high, connect two resistors in series from 8V source to ground
connect the collector of an NPN transistor to enable
connect base of said transistor to junction of two resistors
connect emitter to ground.
when 8 V is present, the transistor will be ON, pulling enable low, boost OFF
when 8 V is not present, transistor will be OFF, letting enable go high, boost ON
add a capacitor at SYS to smooth transition
resistor values not critical
from 8V to base, about 10k
from base to ground, about 1K
transistor - any small signal device useful as a switch, like 2N2222A or 2N3019