treewide: Replace TRUE/FALSE with true/false

C language standard uses true/false for the boolean type.
Let's switch to that for better language compatibility.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Tested-by: Samuel Holland <samuel@sholland.org>