uboot-config.bbclass: Allow choose of U-Boot config for machine

Some machines provide several possible configurations and until now
there was no easy way for user to override the default setting.

This class provides a system similar to PACKAGECONFIG but for
U-Boot. The format is:

UBOOT_CONFIG ??= <default>
UBOOT_CONFIG[foo] = "config,images"

There are two possible parameters:

 - config: it is used to set UBOOT_MACHINE
 - images: it is used to append onto IMAGE_FSTYPES

Below there's an usage example:

,----[ i.MX6Q SABRE AUTO based example ]
| UBOOT_CONFIG ??= "sd"
| UBOOT_CONFIG[sd] = "mx6qsabreauto_config,sdcard"
| UBOOT_CONFIG[eimnor] = "mx6qsabreauto_eimnor_config"
| UBOOT_CONFIG[nand] = "mx6qsabreauto_nand_config,ubifs"
| UBOOT_CONFIG[spinor] = "mx6qsabreauto_spinor_config"
`----

User can, from local.conf or environment, use UBOOT_CONFIG=nand and
override the default setting, as:

,----[ Override example from command line ]
| MACHINE=imx6qsabreauto UBOOT_CONFIG=nand bitbake core-image-base
`----

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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