Makefile: Fix calling make with V=1

Calling 'make V=1 all' on Ubuntu 18.04 with gcc version 9.2.1 and GNU Make
version 4.1 fails on error:

    scripts/Kbuild.include:220: *** Recursive variable 'echo-cmd' references itself (eventually).  Stop.

As a workaround expand 'echo-cmd' variable via 'call' construction instead
of expanding it directly.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reported-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Fixes: ae897022 ("Makefile: Fix u-boot-nodtb.bin target")