itest: fix result of string compares

The implementation of the string compare function of the "itest"
command was weird, as only the length of the shortest argument was
included in the compare, with the result that something like
"itest.s abd == abddef" would return TRUE.  Fix this.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>