uboot: add option to optionally depend on host-openssl

uboot: add option to optionally depend on host-openssl

Some U-Boot board configurations have CONFIG_FIT_SIGNATURE=y. In this
case, when U-Boot builds its tools, it requires OpenSSL support on the
host system.

Since we clearly don't want to unconditionally depend on host-openssl,
we introduce a new option to explicitly enable the dependency on
host-openssl. This is exactly identical to the existing
BR2_TARGET_UBOOT_NEEDS_DTC, for the dependency on host-dtc. It is
therefore the responsibility of the user to enable this option if his
board configuration uses CONFIG_FIT_SIGNATURE=y.

Necessary to fix bug #9316.

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