build-vm: standalone build is a must, too

The problem was spotted by Alexander Bandura:
bin/tar2vm wasn't present in the generated profile.

I considered extending features.in/Makefile to include
bin/ alongside lib/ but that would make the helper's location
unpredictable (unless BUILDDIR is specified explicitly) so
restricting sudoers would be harder; worse yet, the copied file
would come with write access for the user building an image.

The implications in restricted case are complex enough anyways
so the recommended implementation would only include a fixed
readonly location like /usr/share/mkimage-profiles/bin/tar2vm
as laid out in doc/vm.txt, and that means it's in the metaprofile
not a generated profile.

features.in/build-vm/generate.mk

@@ -0,0 +1,4 @@
+# copy tar2vm helper into generated profile to enable standalone builds
+
+all:
+	@install -pD $(MKIMAGE_PROFILES)/bin/tar2vm $(BUILDDIR)/bin/tar2vm

features.in/syslinux/generate.mk

@@ -26,7 +26,6 @@ endif
 # 01defaults.cfg is included indefinitely
 SYSLINUX_CFG := $(SYSLINUX_CFG) $(notdir $(SUBPROFILES)) defaults
 
-### have to operate BUILDDIR, not pretty...
 DSTDIR := $(BUILDDIR)/stage1/files/syslinux/.in
 
 # we can do SYSLINUX_{CFG,MODULES,FILES}

main.mk

@@ -6,8 +6,6 @@
 # --- in BUILDDIR
 # 4. build subprofiles and subsequently an image
 
-MKIMAGE_PROFILES := $(dir $(lastword $(MAKEFILE_LIST)))
-
 # deal with one target at a time
 IMAGE_TARGET := $(firstword $(MAKECMDGOALS))#	ve/generic.tar.gz
 ifeq (./,$(dir $(IMAGE_TARGET)))#		convenience fallback
@@ -26,6 +24,8 @@ else
 BUILDDIR_PREFIX ?= mkimage-profiles.build/$(IMAGE_CONF).$(ARCH)
 endif
 
+export MKIMAGE_PROFILES := $(realpath $(dir $(lastword $(MAKEFILE_LIST))))
+
 # preferences
 -include $(HOME)/.mkimage/profiles.mk