openssl: Use mkhash for STAMP_CONFIGURED

The current way of creating a STAMP_CONFIGURED filename for OpenSSL can
lead to an extremely long filename that makes touch unable to create it,
and fail the build.

Use mkhash to produce a hash against OPENSSL_OPTIONS which creates a
shortert stamp file,

Fixes #572

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

package/libs/openssl/Makefile

@@ -168,7 +168,7 @@ else
   endif
 endif
 
-STAMP_CONFIGURED := $(STAMP_CONFIGURED)_$(subst $(space),_,$(OPENSSL_OPTIONS))
+STAMP_CONFIGURED := $(STAMP_CONFIGURED)_$(shell echo $(OPENSSL_OPTIONS) | mkhash md5)
 
 define Build/Configure
 	[ -f $(STAMP_CONFIGURED) ] || { \