common: update: fix an "unused" warning against update_flash()

Since update_flash() is used only in update_tftp(), it should be
guarded with appropriate config options.

After the commit 3149e524, common/update.c will be built under
either CONFIG_UDATE_TFTP, CONFIG_DFU_TFTP or CONFIG_UPDATE_FIT.
Since CONFIG_UPDATE_FIT, hence fit_update(), doesn't rely on
update_flash(), the compiler may cause an "unused" warning if
CONFIG_UPDATE_FIT=y and CONFIG_UPDATE_TFTP=n and CONFIG_DFU_TFTP=n.

This is, for example, the case for sandbox defconfig where
EFI_CAPSULE_FIRMWARE_FIT is enabled for test purpose.

Fixes: 3149e524 ("common: update: add a generic interface for FIT
       image")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>