Makefile.toolchain.zephyr: Test CONFIG_TOOLCHAIN_VARIANT robustly.

Current code tests treats "" (quotes included) as an empty value for that
varible (as coming from Kconfig). However, that file also used to
integrate with build systems for 3rd-party components, e.g. as examplified
by samples/static_lib, and in that case it will be just an empty string.
So, first strip surrounding quotes, then test for empty string.

samples/static_lib: Update for recent Makefile.toolchain.zephyr changes.

Makefile.toolchain.zephyr is 2-pass now, so needs to be included twice.
ARCH needs to be defined *before* including it. Otherwise, just used
CROSS_COMPILE and TOOLCHAIN_CFLAGS set by it.

Change-Id: I56e963d0d107c77390395682d60400cf6ca62337
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>