branding-processor-sdk: add custom MKUBIFS_ARGS
branding-processor-sdk: add custom MKUBIFS_ARGS

* processor-sdk often includes content not yet upstreamed, and
  therefore the rootfs image size will exceed upstream limits.
* Override the MKUBIFS_ARGS here so that the machine configuration
  files do not need to be duplicated in this layer.

Signed-off-by: Jacob Stiffler <0ec61db87821c27ca8ee6f3e24c6f93fdbb785d4@ti.com>

rcn-ee/meta-processor-sdk,rcn-ee/meta-processor-sdk,rcn-ee/meta-processor-sdk