find7-common: init: fix unified layout detection

 * ^ is a bitwise operator, not "power of" and thus the result is "21"
 * include math.h and use the pow function which fixes it

BEFORE:
 size is 28716384768
 border is 21

AFTER:
 size is 28716384768
 border is 7000000000

Change-Id: Ic557157dcb4149ecdeb823dc2747c6f7f65d2ff0