build: further prevent non-cbmk-work-directory

this is a follow-up to the previous commit

again, there's no posix way to check the path to the
file at argument 0, because readlink (utility) isn't
defined in posix (the C function is defined, but not
the utility included on many unices)

check whether "build" (file) exists, and whether it
is a symlink; if the latter, then we are definitely
not in the cbmk work directory!

Signed-off-by: Leah Rowe <leah@libreboot.org>

build

@@ -7,7 +7,7 @@
 
 set -u -e
 
-if [ "./${0##*/}" != "${0}" ]; then
+if [ "./${0##*/}" != "${0}" ] || [ ! -f "build" ] || [ -L "build" ]; then
 	printf "You must run this in the cbmk work directory.\n" 1>&2
 	exit 1
 fi