system: Make system_can_boot_ap return true for AC-only systems
system: Make system_can_boot_ap return true for AC-only systems

This patch makes system_can_boot_ap return true for systems which
can be powered only by a fixed AC power (e.g. barrel jack AC adapter).
Such systems do not need to check a battery percentage or AC power.

Signed-off-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

BUG=none
BRANCH=none
TEST=buildall

Change-Id: Icd59b508e944c43253d416da78d0f1a87fedb13d
Reviewed-on: https://chromium-review.googlesource.com/1301935
Commit-Ready: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Tested-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Reviewed-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec