block: rename the flag of emmc_disk to is_rk_disk

block: rename the flag of emmc_disk to is_rk_disk

flag of emmc_disk was used to indicate that if the main
disk is for rockchip specific as we need to use rk specific
partition layout. But this name is vague and pointless since
the main disk could be sdmmc as we support sdmmc bootup.
Moreover, we could support more types of storage devices, for
instance, NVMe, UFS etc. Let's rename it to better reflect what
it is here.

Change-Id: Ibdeb4a930b97767d796ddfc050ee9d4d0ddeceb7
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>