[POWERPC] Move bootwrapper's strchr() and strncmp() from .h to string.S

Currently the bootwrapper has implementations of strchr() and
strncmp(), but they're inlines in flatdevtree_env.h, rather than in
string.S with all the rest of the string functions.  This moves
them to string.S.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>