test/py: net_boot: Add test cases for net boot

Add tests for booting image using tftpboot/pxe boot commands, tftpboot
boot case loads the FIT image into DDR and boots using bootm command
whereas pxe boot cases downloads the pxe configuration file from the
TFTP server and interprets it to boot the images mentioned in the pxe
configurations file.
This test relies on boardenv_* containing configuration values including
the parameter 'pattern'. tftpboot/pxe boot cases boots the Linux till the
boot log pattern value is matched. For example, if the parameter
'pattern' is defined as 'login:', it will boot till login prompt.

Signed-off-by: Love Kumar <love.kumar@amd.com>
Tested-by: Tom Rini <trini@konsulko.com>