Unify the style of _MakeRamdisk for ramdisk format

RamdiskFormat class is introduced by commit : f3f842b6
But it seems _MakeRamdisk() related codes are also needed, then
both of the code style are same.

This patch takes below 2 changes:
1) adds new commom function _GetRamdiskFormat()
2) unfiy the code logic of _MakeRamdisk() and its related code logic.

Change-Id: Ibd4932a6050fbac15fcd741c70dd7854c12e887d
Signed-off-by: jiajia tang <tangjiajia@xiaomi.com>