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>

Makefile

@@ -1201,7 +1201,7 @@ certtool: ${CRTTOOL}
 
 .PHONY: ${CRTTOOL}
 ${CRTTOOL}:
-	${Q}${MAKE} PLAT=${PLAT} USE_TBBR_DEFS=${USE_TBBR_DEFS} COT=${COT} --no-print-directory -C ${CRTTOOLPATH}
+	${Q}${MAKE} PLAT=${PLAT} USE_TBBR_DEFS=${USE_TBBR_DEFS} COT=${COT} OPENSSL_DIR=${OPENSSL_DIR} --no-print-directory -C ${CRTTOOLPATH}
 	@${ECHO_BLANK_LINE}
 	@echo "Built $@ successfully"
 	@${ECHO_BLANK_LINE}
@@ -1267,7 +1267,7 @@ enctool: ${ENCTOOL}
 
 .PHONY: ${ENCTOOL}
 ${ENCTOOL}:
-	${Q}${MAKE} PLAT=${PLAT} BUILD_INFO=0 --no-print-directory -C ${ENCTOOLPATH}
+	${Q}${MAKE} PLAT=${PLAT} BUILD_INFO=0 OPENSSL_DIR=${OPENSSL_DIR} --no-print-directory -C ${ENCTOOLPATH}
 	@${ECHO_BLANK_LINE}
 	@echo "Built $@ successfully"
 	@${ECHO_BLANK_LINE}

docs/getting_started/build-options.rst

@@ -721,6 +721,10 @@ Common build options
   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
 --------------------
 

make_helpers/defaults.mk

@@ -308,3 +308,6 @@ RAS_TRAP_LOWER_EL_ERR_ACCESS	:= 0
 
 # Build option to create cot descriptors using fconf
 COT_DESC_IN_DTB			:= 0
+
+# Build option to provide openssl directory path
+OPENSSL_DIR			:= /usr