u-boot: support building for mkimage

To build mkimage for the host (which is needed to build an uImage of
the kernel), it is not necessary to configure U-Boot, and therefore to
have a particular board selected.

Therefore, this commit:

 * Adds a verification at U-Boot configure step that a U-Boot board
   name has been defined

 * Sets a default U-Boot version if none has been specified, so that
   even when U-Boot isn't selected but we want to build mkimage for
   the host, a particular U-Boot version is picked.

 * Make the host mkimage target depend only on U-Boot being
   downloaded/extracted/patched, and the target mkimage/fw_printenv
   targets depend on U-Boot being fully configured.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>