core: use $(CURDIR) to set TOPDIR

This changes saves a shell call and uses a variable automatically set
by make [1].

[1] http://www.gnu.org/software/make/manual/make.html#Quick-Reference

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -69,7 +69,7 @@ endif
 .NOTPARALLEL:
 
 # absolute path
-TOPDIR := $(shell pwd)
+TOPDIR := $(CURDIR)
 CONFIG_CONFIG_IN = Config.in
 CONFIG = support/kconfig
 DATE := $(shell date +%Y%m%d)