Add system_can_boot_ap API

This API checks battery charge level and current power supply
to determine whether the AP has enough power to boot or not.

BUG=b:38462249
BRANCH=none
TEST=make buildall

Change-Id: I489f7ea92f230701b8f18c94d3e698aad90b4a03
Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/517272

include/system.h

@@ -529,4 +529,12 @@ static inline void system_print_extended_version_info(void)
 {
 }
 #endif
+
+/**
+ * Check if the system can supply enough power to boot AP
+ *
+ * @return true if the system is powered enough or false otherwise
+ */
+int system_can_boot_ap(void);
+
 #endif  /* __CROS_EC_SYSTEM_H */