Add the REBOOT kconfig option, along with the sys_reboot() API.
This infrastructure is not enough to actually perform a reboot.
Architecture/platform code must be provided as well, in the form of a
sys_arch_reboot() function.
Change-Id: I5b2b15855ff06453f2764f3e3b3b7d6a4a078723
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>