tfa-layerscape: set BUILD_DEVICES

tfa-layerscape: set BUILD_DEVICES

Currently, tfa-layerscape packages are being included in the individual
profile DEVICE_PACKAGES but using the feature that allows skipping their
inclusion in the end image package list if prefixed with a tilde(~) which
was added in:
377b66990b97 ("build: introduce support to declare skip package")

But it not added to Image Builder so currently trying to build layerscape
device images in Image Builder will fail with:
ERROR: '~trusted-firmware-a-ls1012a-frdm' is not a valid world dependency, format is name(@tag)([<>~=]version)

So, instead of having to rely on support for skipping package installation
and declaring the individual TFA packages in DEVICE_PACKAGES we can just
do what other targets do and set BUILD_DEVICES so that TFA packages are
automatically set.

Fixes: #18412
Link: https://github.com/openwrt/openwrt/pull/18462
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 03ab770da5a469e81a659574a66dd2f7275d431e)