uboot-config.bbclass: Allow multiple U-Boot config for machine
This adds support to build multiple U-Boot configs for a machine; this
is useful when we have support for different media boots which require
different U-Boot configuration (e.g: eMMC and NAND).
Below there's an usage example:
,----[ i.MX6Q SABRE AUTO based example ]
| UBOOT_CONFIG ??= "sd eimnor nand spinor"
| 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"
`----
Signed-off-by: Chunrong Guo <B40290@freescale.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
