Add boot.img substitution to the build_mixed script.

Bug: 131879395
Test: ./build_mixed -b boot-debug.img <system> <device> <out>
Change-Id: I2877e79522b5836e876c87f38221ca6890c41ce6
Merged-In: I2877e79522b5836e876c87f38221ca6890c41ce6
(cherry picked from commit 22da1576b115eab873519f5f31a531aec836ccd6)
1 file changed