Libpayload: default DESTDIR for 'make install'

If you would try download FILO via svn, then you probably
get error message about libpayload install. This enables
manually installing libpayload in legacy style :)

Change-Id: I9f52be939303c5913611f21477d681e11d286382
Signed-off-by: Tadas Slotkus <devtadas@gmail.com>
Reviewed-on: http://review.coreboot.org/102
Tested-by: build bot (Jenkins)
Reviewed-by: Frank Vibrans III <frank.vibrans@amd.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
diff --git a/payloads/libpayload/Makefile.inc b/payloads/libpayload/Makefile.inc
index 08f7c9f..03862ca 100644
--- a/payloads/libpayload/Makefile.inc
+++ b/payloads/libpayload/Makefile.inc
@@ -33,6 +33,7 @@
 
 ARCHDIR-$(CONFIG_TARGET_I386)    := i386
 ARCHDIR-$(CONFIG_TARGET_POWERPC) := powerpc
+DESTDIR ?= install
 
 real-target: lib