musl: improve crypt() size hack

Instead of silently downgrading any non-MD5 crypt() request to DES,
cleanly fail with return NULL and errno = ENOSYS. This allows callers
to notice the missing support instead of the unwanted silent fallback
to DES.

Also add a menuconfig toolchain option to optionally disable the crypt
size hack completely. This can be probably made dependant on SMALL_FLASH
or a similar feature indicator in a future commit.

Ref: https://github.com/openwrt/openwrt/pull/1331
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

toolchain/Config.in

@@ -262,6 +262,7 @@ choice
 endchoice
 
 source "toolchain/uClibc/Config.in"
+source "toolchain/musl/Config.in"
 
 comment "Debuggers"
 	depends on TOOLCHAINOPTS