Add support of partition labels for U-Boot by analogy with FNG Boot.

Add support of partition labels for U-Boot by analogy with FNG Boot.

Corresponding code from FNG Boot sources was ported to current version of U-Boot.
A new configuration option is added to U-Boot to determine a storage partition with U-Boot environment by label during boot: CONFIG_ENV_FAT_DEVICE_LABEL.
Herewith the storage device, on which a partition with requested label will be searched, still should be specified via CONFIG_ENV_FAT_DEVICE_AND_PART.

For example:
CONFIG_ENV_FAT_DEVICE_AND_PART="0"
CONFIG_ENV_FAT_DEVICE_LABEL="config"

Notes and limitations:

• searching routine is not case sensitive (i.e. BOOT and boot will be treated equally);
• only storages with MBR partition table are supported (i.e. storages with GPT partition table are not supported).