The gitpkgv class we are using to set UBOOT_VERSION is located in
meta-openembedded. We do not want to depend on meta-openembedded to
keep meta-ti-bsp as light as possible, and the naming of UBOOT_VERSION
is not a requirement, just a nice to have. Dynamic layers allow us to
use the class if it is available in the build, but not require it.
Signed-off-by: Ryan Eatmon <reatmon@ti.com>