test/py: Add test support for three stage boot

Current pytest only support upto 2 stage boot;
Some boards like TI K3 am6/J7 boards use 3 stage
boot. This patch adds u_boot_spl2 to be able to
handle the 3-stage boot case. User needs to set
"env__spl2_skipped" in u_boot_boardenv config
file to use this support. By default it is set
to TRUE.

Signed-off-by: Yan Liu <yan-liu@ti.com>
Signed-off-by: Yan Liu <yan-liu@ti.com>