efi_firmware: set EFI capsule dfu_alt_info env explicitly

efi_firmware: set EFI capsule dfu_alt_info env explicitly

The current implementation of EFI capsule update uses set_dfu_alt_info() to
set the dfu_alt_info environment variable with the settings it requires.
However, set_dfu_alt_info() is doing this for all DFU operations, even
those unrelated to capsule update.

Thus other uses of DFU, such as DFU boot which sets its own value for the
dfu_alt_info environment variable, will have that setting overwritten with
the capsule update setting. Similarly, any user defined value for the
dfu_alt_info environment variable would get overwritten when any DFU
operation was performed, including simply performing a "dfu 0 list"
command.

The solution is stop using the set_dfu_alt_info() mechanism to set the
dfu_alt_info environment variable and instead explicitly set it to the
capsule update's setting just before performing the capsule update's DFU
operation, and then restore the environment variable back to its original
value.

This patch implements the explicit setting and restoring of the
dfu_alt_info environment variable as part of the EFI capsule update
operation.

The fix is fully implemented in a subsequent patch that removes the capsule
update dfu_alt_info support in set_dfu_alt_info().

Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>