By default, instead of spamming the build output with one warning per
file where __assert.h is included when CONFIG_ASSERT=y, only display a
warning at the end of the build.
Also limit the range of CONFIG_ASSERT_LEVEL between 0 and 2.
Change-Id: I95ffd1bcec9535de1afabc047814e5c6f5b9c2c1
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>