Makefile, doc: Make OPENSSL_DIR variable as build option for tools

Openssl directory path is hardcoded to '/usr' in the makefile
of certificate generation and firmware encryption tool using
'OPENSSL_DIR' variable.

Hence changes are done to make 'OPENSSL_DIR' variable as
a build option so that user can provide openssl directory
path while building the certificate generation and firmware
encryption tool.

Also, updated the document for this newly created build option

Change-Id: Ib1538370d2c59263417f5db3746d1087ee1c1339
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index bfc50df..630d861 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -721,6 +721,10 @@
   bit, to trap access to the RAS ERR and RAS ERX registers from lower ELs.
   This flag is disabled by default.
 
+- ``OPENSSL_DIR``: This flag is used to provide the installed openssl directory
+  path on the host machine which is used to build certificate generation and
+  firmware encryption tool.
+
 GICv3 driver options
 --------------------