POST: make env test flags fetching optional

Some boards have the environment variables defined in a slow EEPROM. post_run
accesses these environment variables to define which tests have to be run (in
post_get_flags). This is very slow before the code relocation on some boards
with a slow I2C EEPROM for environement variables.

This patch adds a config option to skip the fetching of the test flags in the
environment variables. The test flags assigned to the tests then only are the
ones statically defined for the test in post/tests.c.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>