Remove separate Linux implementation

Linux /dev/urandom implementation now folded into main file.

Main file can be forced to use own pseudo-RNG by defining RANDLIB_PRNG -
used for unit tests, and could maybe also be used on simulator.

Author: kjbracey

Makefile

@@ -1,15 +1,4 @@
-
-# Check if we are compiling for Linux using GCC
-ifneq (,$(findstring gcc,$(CC)))
-TARGET_IS_LINUX := $(shell $(CC) -dM -E - < /dev/null | grep __linux >/dev/null && echo "yes" || echo "no")
-endif
-
-ifeq (yes,$(TARGET_IS_LINUX))
-SRCS := $(wildcard linux/*)
-else
 SRCS := $(wildcard source/*)
-endif
-
 LIB := librand.a
 EXPORT_HEADERS := mbed-client-randlib