[U-Boot] strtonum() is gawk specific extension

[Vasili Galka]

The following patch "Check that u-boot.bin size looks correct":
http://patchwork.ozlabs.org/patch/355151/
Uses strtonum() function which is gawk specific extension:
https://www.gnu.org/software/gawk/manual/html_node/Strtonum-Function.html

As a result, currenlty, u-boot/master generates build errors on systems
with alternative awk implementations e.g. I get:

awk: line 2: function strtonum never defined

Either gawk shall be required and explicitly used or the
implementation changed.

Best,
Vasili