u-boot: introduce UBOOT_INITIAL_ENV

It defaults to ${PN}-initial-env, no functional changes with current
implementation, but this allows it to be changed in individual u-boot
recipes.

If UBOOT_INITIAL_ENV is empty, then no initial env would be compiled/
installed/deployed, set ALLOW_EMPTY_${PN}-env = "1".

The major purpose for introducing this, is that the users might have
some scripts on targets like:
```
/sbin/fw_setenv -f /etc/u-boot-initial-env
```

and it should be able to run against a identical path generated by
different u-boot recipes.

Signed-off-by: Ming Liu <ming.liu@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit fb7e8b6b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>