build: Switch to TF-M provided NS toolchain

Removed the previous toolchain dependency and the NS toolchain exported
by TF-M is used instead. The cross compilation flags are set up by
this toolchain file (e.g. -mcpu). The compilation options are set by
FRI, these options are the previously used options from the removed
toolchain and the options used in TF-M merged.

Signed-off-by: Gergely Kovacs <Gergely.Kovacs2@arm.com>