lib: Kconfig dependencies for pseudo-random library

drivers/rng/sandbox_rng.c requires rand() to be defined but configuration
option CONFIG_CONFIG_LIB_RAND selected in drivers/rng/Kconfig does not
exist.

test/lib/test_aes.c requires rand() to be defined.

Fix the selection criteria for choice "Pseudo-random library support type".

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>