package/uboot-tools: depend on u-boot when selected

Currently, the envimage creation logic only depends on u-boot when the
user does not specify a custom envimage source via
BR2_PACKAGE_HOST_UBOOT_TOOLS_ENVIMAGE_SOURCE. This assumes that the
user-provided envimage source is not coming from the u-boot source
tree.

But especially given the fact that the envimage creation logic used to
be part of the u-boot package, this is a realistic scenario: users may
have provided a value of BR2_PACKAGE_HOST_UBOOT_TOOLS_ENVIMAGE_SOURCE
based on $(UBOOT_DIR), e.g.:

    $(UBOOT_DIR)/board/foo-vendor/bar-board/env.txt

Therefore, always add the u-boot dependency if u-boot is selected, for
either case of custom or default envimage source.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[Thomas: re-organize code a bit.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>