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>