base: improve LICENSE_FLAGS_DETAILS output

Don't prefix the output of LICENSE_FLAGS_DETAILS with "For further
details, see" so that recipes can put arbitrary text in their license
details instead of being limited to a specific sentence structure.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>