package/uboot-tools: migrate BR2_TARGET_UBOOT_ENVIMAGE from U-Boot pkg

package/uboot-tools: migrate BR2_TARGET_UBOOT_ENVIMAGE from U-Boot pkg

Migrating the support for this feature to uboot-tools to gain the
ability to build env files when BR2_TARGET_UBOOT isn't selected.

When _ENVIMAGE_SOURCE is not set, we generate a default environment.
However, this default depends on the U-Boot configuration. Therefore,
this can only be done if uboot itself is built as well, and
host-uboot-tools needs to depend on uboot.

For the same reason, the commands for creating the environment have to
be adapted a little. Take this occasion to drastically simplify them.

Note: This patch creates a circular dependency with uboot until the
similar migration patch is merged for uboot scripts

Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>