Make linux kernel string funcs available to tools

isspace() and strim() are not in the typical user-mode string.h, so
put them in a separate compilation unit so that they can be built into
tools that need them independent of the other common string functions.

This allows code shared by u-boot and the linux user-mode tools to link.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>