include/trusted-firmware-a.mk: support LTS releases

include/trusted-firmware-a.mk: support LTS releases

TF-A LTS releases have an lts- prefix in their version tag. Introduce a
PKG_LTS variable to support these.

As the non-LTS tarbals do not contain the version prefix in the
directory, we need different PKG_BUILD_DIR for each variant:

➜ tar --list --file dl/trusted-firmware-a-v2.12.tar.gz  | head -n1
trusted-firmware-a-2.12/
➜ tar --list --file dl/trusted-firmware-a-lts-v2.12.1.tar.gz  | head -n1
trusted-firmware-a-lts-v2.12.1/

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>