Add custom version option, to enable easily specifying an official OP-TEE version to use. This is similar to what is done in other packages, such as linux, uboot or arm-trusted-firmware. Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com> Cc: Étienne Carrière <etienne.carriere@foss.st.com> Signed-off-by: Julien Olivain <ju.o@free.fr>