[U-Boot] Export CFI Flash to the MTD and support it in UBI

Following patches allow for CFI Flash support in UBI and other MTD based parts of U-Boot. This is done by adding cfi_mtd driver which exports CFI Flash to the MTD layer.

First patch adds cfi_mtd driver to the U-Boot. Second cleans printf() in MTD layer. The last one adds proof-of-concept CFI Flash support to the UBI.

The baseline of the following patches is the 'testing' branch in the 'u-boot-ubi' repo.

Best regards, Piotr Ziecik