api: Export API structure address as an environment variable

This patch makes the U-Boot api export its structure address as an environment
variable, so it can be used to directly hint FreeBSD's loader of api's location.

The relevant FreeBSD loader change is currently under review at:
https://reviews.freebsd.org/D5492



Signed-off-by: Stanislav Galabov <sgalabov@gmail.com>