_exports.h: export standard memory/string handling functions

_exports.h: export standard memory/string handling functions

The current list of exported functions lacks quite a few bog-standard C
library functions that we might as well expose, since U-Boot certainly
has them implemented anyway. There's no reason a standalone
application should have its own strlen() implementation or link in a
copy from some tiny libc.

For a customer's standalone app, this means it goes from 95K to 10K.
More importantly, we can ditch the custom toolchain including a
newlibc used to build the standalone app and just use the same
toolchain as used to build u-boot itself.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>