fastboot: check for alias when looking up partition by name

Implement an alias name check for devices where GPT limitations prevent
user-friendly partition names such as "boot", "system" and "cache". Or,
where the actual partition name doesn't match a standard partition name
used commonly with fastboot.

To set an alias, add an environment setting as follows:
fastboot_partition_alias_<alias partition name>=<actual partition name>

Example: fastboot_partition_alias_boot=LNX

Signed-off-by: Michael Scott <michael.scott@linaro.org>
Acked-by: Steve Rae <srae@broadcom.com>
Cc: Steve Rae <srae@broadcom.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>