This handler tries to read from a file descriptor with 'write only'
flags and fails. Add read permissions for the file, so the handler can
read the partition layout.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

With this patch the barebox_update command will be extended by the possibility
to flash the MLO to eMMC devices.

The MLO will be flashed to the following addresses:
0x00000
0x20000
0x40000
0x60000

Because the first 512 Bytes of the MLO are reserved for the CHSETTINGS header
and this only use ~80 Bytes, there is space for the partition table in the
header.

The command will overwrite the bootstrap code area and will hold the
partition table and the Boot signature.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>