UBOOT_CONFIG variable can hold multiple values.
Redefined base build steps to support that.
As a result - multiple imx-boot binaries will be generated:
    imx-boot-seco-mx8mm-sd.bin-flash_evk
    imx-boot-seco-mx8mm-sd.bin-flash_evk.tar.gz
    imx-boot-seco-mx8mm-sd_no_console.bin-flash_evk
    imx-boot-seco-mx8mm-sd_no_console.bin-flash_evk.tar.gz

The short link to the u-boot tar.gz is also created for all variants
(with a little hacky name creation).