Constify getenv(), setenv() and hash code functions

This is needed to get rid of build warnings like

main.c:311: warning: passing argument 2 of 'setenv' discards qualifiers from pointer target type

which result from commit 09c2e90c "unify version_string".

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Andreas Biemann <andreas.devel@googlemail.com>