mksh: changed to build on cross stage if requested
linux: changed to enable support for scripts starting with #! by default

Note:

This option was introduced in 3.10 and is quite important for initramfs init
scripts to work, thus enabling it by default is a good thing to do!

http://cateee.net/lkddb/web-lkddb/BINFMT_SCRIPT.html

Thanks Turl for pointing testing and pointing me to that option!