Rename the boot image for avb validation

Starting from http://go/aog/1328118, we chained the 1st GKI
boot image into vbmeta. However, this fails avb validation;
because the avbtool constructs the image path based by using
"partition name + ext".

This cl works around the issue by renaming boot-5.4.img to
boot.img, so avbtool can find the image correctly.

Bug: 159656873
Test: run validate target file
Change-Id: I577226596e139e5b5f3e6ca3c28ced02431ca392