lib: rand: introduce new configs: CONFIG_LIB_RAND and CONFIG_LIB_HW_RAND

lib: rand: introduce new configs: CONFIG_LIB_RAND and CONFIG_LIB_HW_RAND

New configs:
- CONFIG_LIB_RAND    - to enable implementation of rand library in lib/rand.c
- CONFIG_LIB_HW_RAND - to enable hardware based implementations of lib rand

Other changes:
- add CONFIG_LIB_RAND to boards configs which needs rand()
- put only one rand.o dependency in lib/Makefile

CONFIG_LIB_HW_RAND should be defined for drivers which implements rand library
(declared in include/common.h):
- void srand(unsigned int seed)
- unsigned int rand(void)
- unsigned int rand_r(unsigned int *seedp)

Signed-off-by: Przemyslaw Marczak <[email protected]>
Cc: Michael Walle <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Masahiro Yamada <[email protected]>