efi: Define set_dfu_alt_info() for boards with UEFI capsule update enabled

efi: Define set_dfu_alt_info() for boards with UEFI capsule update enabled

Currently, there are a bunch of boards which enable the UEFI capsule
update feature. The actual update of the firmware images is done
through the dfu framework which uses the dfu_alt_info environment
variable for getting information on the update, like device, partition
number/address etc. The dfu framework allows the variable to be set
through the set_dfu_alt_info function defined by the platform, or if
the function is not defined, it gets the variable from the
environment. Using the value set in the environment is not very
robust, since the variable can be modified from the u-boot command
line and this can cause an incorrect update.

To prevent this from happening, define the set_dfu_alt_info function
when the capsule update feature is enabled. A weak function is defined
which sets the dfu_alt_info environment variable by getting the string
for the variable from the platform.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>