build: remove the variable NM in gen_ll_addressable_symbols.sh

With LTO activated, the buildman tools failed with an error on my
configuration (Ubuntu 20.04, stm32mp15_trusted_defconfig) with the error:

../arm-linux-gnueabi/bin/nm:
	scripts/gen_ll_addressable_symbols.sh: file format not recognized

It seems the shell variable initialization NM=$(NM) is not correctly
interpreted when shell is started in the Makefile, but I have not this
issue when I compile the same target without buildman.

I don't found the root reason of the problem but I solve it by
providing $(NM) as script parameter instead using a shell variable.

The command executed is identical:

cmd_keep-syms-lto.c := NM=arm-none-linux-gnueabihf-gcc-nm \
u-boot/scripts/gen_ll_addressable_symbols.sh arch/arm/cpu/built-in.o \
.... net/built-in.o >keep-syms-lto.c

cmd_keep-syms-lto.c := u-boot/scripts/gen_ll_addressable_symbols.sh \
arm-none-linux-gnueabihf-gcc-nm arch/arm/cpu/built-in.o \
... net/built-in.o > keep-syms-lto.c

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>