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".

(From OE-Core rev: 314885b16e5d26d27d46a4bfb0d581b27a03b8fe)

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>

meta/classes/kernel-uimage.bbclass

@@ -3,7 +3,7 @@ inherit kernel-uboot
 python __anonymous () {
     if "uImage" in d.getVar('KERNEL_IMAGETYPES'):
         depends = d.getVar("DEPENDS")
-        depends = "%s u-boot-mkimage-native" % depends
+        depends = "%s u-boot-tools-native" % depends
         d.setVar("DEPENDS", depends)
 
         # Override KERNEL_IMAGETYPE_FOR_MAKE variable, which is internal