buildroot

linux: add BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL

Some Linux kernel configuration options (such as
CONFIG_SYSTEM_TRUSTED_KEYRING) require building a host program called
extract-cert, which itself needs OpenSSL.

Users having OpenSSL installed on their system won't see a problem,
but users who don't have OpenSSL installed will get a build
failure. This commit adds a new option that allows users to indicate
that their Linux configuration requires building host-openssl.

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