UBUNTU: SAUCE: fix build error with CONFIG_BLK_DEV_INITRD=n

UBUNTU: SAUCE: fix build error with CONFIG_BLK_DEV_INITRD=n

The original aynchronous boot patch (UBUNTU: SAUCE: Make populate_rootfs asynchronous)
did not take into consideration the case when CONFIG_BLK_DEV_INITRD=n,
e.g., populate_rootfs_domain becomes undefined. Therefore, add it to
noinitramfs.c where its use is benign.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Original-patch-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Surbhi Palande <surbhi.palande@canonical.com>
Signed-off-by: Leann Ogasawara <leann.ogasawara@canonical.com>