init: Add kernel option to force usage of tmpfs for rootfs

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

-------------------------------------------------

This patch adds the new option initramtmpfs for the kernel command line, to
force usage of tmpfs instead of ramfs as filesystem for rootfs.

This option should be used when the initial ram disk contains xattrs, as
only tmpfs supports them.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Tianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>