codebase: Change makefile $(shell pwd) commands to $(CURDIR)

- Change the makefile command $(shell pwd) to $(CURDIR) to find the
current directory without going out to the shell.

Change-Id: I4890eba6129630acd2883b92de77308d39949443
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13967
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/bimgtool/Makefile

@@ -1,4 +1,4 @@
-obj ?= $(shell pwd)
+obj ?= $(CURDIR)
 
 HOSTCC ?= gcc
 CFLAGS ?= -g