package/linux-firmware: fix build failure when no firmware item selected
The latest change in Linux firmware handling breaks the build if no
specific firmware item is selected below BR2_PACKAGE_LINUX_FIRMWARE.
The firmware archive is only created if at least one firmware
sub-category is selected; the installation step wants to unpack the
archive unconditionally.
Use the same condition to control install command definition as in the
build step.
Signed-off-by: Andreas Ziegler <br015@umbiko.net>
[yann.morin.1998@free.fr: use a single conditional block]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>