boot/arm-trusted-firmware: build fiptool separately with dependency on host-openssl

boot/arm-trusted-firmware: build fiptool separately with dependency on host-openssl

ATF >= 1.3 builds a host program called fiptool which uses
OpenSSL, so we need to build host-openssl. We could have made it an
optional dependency like U-Boot does, but since most ATF versions are
going to be >= 1.3 in the near future, we simply make host-openssl a
mandatory dependency.

However, the ATF build system is not very good, and you can't easily
pass flags that will affect the build of host programs. Therefore, we
take the approach of building fiptool separately before triggering the
real build process.

It would obviously be better to fix ATF itself, but as usual with
those bootloader packages, we fetch different versions depending on
the platform/configuration, making it difficult to use patches.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/44868961

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>