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 <p.marczak@samsung.com>
Cc: Michael Walle <michael@walle.cc>
Cc: Tom Rini <trini@ti.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>