[sandbox] add propper sandbox detection ...

This patch adds a __SANDBOX__ define to identify compiling for the
sandbox.
When building for sandbox, don't use sync() workaround.

Fixes this error for sandbox on PPC:

CC      common/memsize.o
common/memsize.c:30:38: error: asm/io.h: No such file or directory
common/memsize.c: In function 'get_ram_size':
common/memsize.c:51: warning: implicit declaration of function 'sync'

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

rename linux target to sandbox