kernel build: use INSTALL_MOD_STRIP=1 during modules_install

[thom_nic]

While researching something else I came across the INSTALL_MOD_STRIP=1 option.  For my am3352 kernel I added this option to the `modules_install` command of my build and it took my directory of 300-ish modules from 115MB to 9.6MB! 

Setting CONFIG_DEBUG_INFO=n in my .config reduced the total size by another 600kB (not nearly as significant but worth noting if you're *really* space sensitive.)

I have not yet tested this thoroughly although from what I've read it's innocuous unless you're debugging kernel code.  If anyone can offer rationalization of why this is not the default recommended practice, please chime in! 

Otherwise I'd highly suggest to Olimex this option be added to the wiki instructions.