vexpress-firmware: new package

vexpress-firmware: new package

This commit adds a new package for the SCP (System Control Processor)
firmware for the ARM reference platforms. There will most likely be
other SCP firmware provided by other vendors, but they might be provided
in a different form, so we for now create a vendor-specific package
named vexpress-firmware.

Since this firmware is used for booting, we create the package in boot/
and not in package/.

The package simply installs a single pre-built binary file into the
images directory.

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
[Thomas:
 - Rename the package from scpfirmware to vexpress-firmware.
 - Add a Config.in prompt for the package.
 - Remove the prompt-less Config.in options to select the Git repo and
   version. Since we only support the Vexpress firmware, there's no need
   for configurability here, so we just use the right Git repo/version
   in the .mk file.
 - Use $(INSTALL) -D to install the firmware file.
 - Add license information.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: add hash file, as suggested by Yann.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>