Implement boot device r/w functions for vboot

BUG=None
TEST=Manual

Test procedure:
1. Compile with VBOOT_DEBUG flag on successfully
2. Run u-boot on dev board as follows:
(Assume you have mmc device on dev board)
CrOS> mmc init
CrOS> cros bootdev set mmc 1
CrOS> cros bootdev
(info of mmc1)
CrOS> read mmc 1 0x20000000 1 2
CrOS> md.b 0x20000000
(mmc1 content)
CrOS> cros bootdev read 0x20001000 1 2
CrOS> md.b 0x20001000
(mmc1 content)
CrOS> cmp.b 0x20000000 0x20001000 0x400
(compare the blocks, should be identical)

Review URL: http://codereview.chromium.org/4001006

Change-Id: I339f9fb63862993f8b9ced80d4c27d6bef0c1bc6
8 files changed