Use make functions to convert a text to lower case

We cannot cache the result of ($shell ...) and we have to run these on
every build even if we don't have to re-read the makefiles. Replace it
with make functions $(substr $(substr ... )).

Bug: 38399657
Test: `make` pass, flash images from $OUT and boot device with chain partitions
Merged-In: I6d4ff8e487931ba25008a2a881401a8951d4049e
Change-Id: I6d4ff8e487931ba25008a2a881401a8951d4049e
(cherry picked from commit e615afd6)