u-boot: Add mkenvimage tool

u-boot: Add mkenvimage tool

This utility is used for creation of images containing
usable in run-time U-Boot environment.

As of today this utility is added per-board like here [1]
for Intel Edison board.

[1] http://git.yoctoproject.org/cgit/cgit.cgi/meta-intel-edison/tree/meta-intel-edison-bsp/recipes-bsp/u-boot/u-boot-tools_2014.04.bb

Given there're quite some U-Boot tools that we may want to add later
this recipe name switch from "u-boot-mkimage" to generic "u-boot-tools"
still for compatibility we provide "u-boot-mkimage" with help
of PROVIDES as well as proposed "u-boot-mkenvimage".

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Martin Jansa <martin.jansa@gmail.com>
Cc: Ross Burton <ross.burton@intel.com>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>