petitboot: depend on BR2_PACKAGE_LVM2 for devicemapper explicitly

Previously, we added dependency to petitboot.mk but we were missing
it from Config.in, which buildroot 2016.02 errors out on.

Select LVM2 when petitboot is selected

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
diff --git a/openpower/package/petitboot/Config.in b/openpower/package/petitboot/Config.in
index 08801ec..e5769de 100644
--- a/openpower/package/petitboot/Config.in
+++ b/openpower/package/petitboot/Config.in
@@ -2,6 +2,7 @@
 	bool "petitboot"
 	# petitboot needs udev /dev management
 	depends on BR2_PACKAGE_HAS_UDEV
+	select BR2_PACKAGE_LVM2
 	select BR2_PACKAGE_NCURSES
 	select BR2_PACKAGE_NCURSES_TARGET_PANEL
 	select BR2_PACKAGE_NCURSES_TARGET_FORM