set flashmap install prefix to /usr instead of /usr/local

/usr/local/ does not exist on non-developer images in Chrome OS.

BUG=chromium-os:22304
TEST=Built locally and ensured files are placed in /usr instead of /usr/local.

For x86-generic:
/build/x86-generic/usr/lib/libfmap.so
/build/x86-generic/usr/lib/libfmap.so.0
/build/x86-generic/usr/lib/pkgconfig/fmap.pc
/build/x86-generic/usr/lib/libfmap.so.0.0.0
/build/x86-generic/usr/include/fmap.h
/build/x86-generic/usr/sbin/fmap_encode
/build/x86-generic/usr/sbin/fmap_decode
/build/x86-generic/usr/sbin/fmap_csum

For amd64-generic:
/build/amd64-generic/usr/lib/pkgconfig/fmap.pc
/build/amd64-generic/usr/lib64/libfmap.so
/build/amd64-generic/usr/lib64/libfmap.so.0
/build/amd64-generic/usr/lib64/libfmap.so.0.0.0
/build/amd64-generic/usr/include/fmap.h
/build/amd64-generic/usr/sbin/fmap_encode
/build/amd64-generic/usr/sbin/fmap_decode
/build/amd64-generic/usr/sbin/fmap_csum

Change-Id: I64278c91c4b52f4d89d8d174a579ddb62da3a0c1

Makefile

diff --git a/Makefile b/Makefile
index cc794fe..acbd00e 100644
--- a/Makefile
+++ b/Makefile
@@ -52,7 +52,7 @@
 # OS package manager may set LIBDIR with respect to 32-bit/64-bit environment
 LIBDIR		?= lib
 
-prefix		= /usr/local
+prefix		= /usr
 sbindir		= $(prefix)/sbin
 libdir		= $(prefix)/$(LIBDIR)
 includedir	= $(prefix)/include