Add further -no-pie checks to Rumprun build tools

This builds upon the previous commit to add -no-pie anywhere the
relocatable flag (-Wl,-r) is used to handle compilers that enable -pie
by default (Such as Debian Stretch).

cookfs: warn if image is >1MB.

rename genfs to cookfs

rumprun-bake: use libexec/readelf -- it's always available

Saves from having to set $READELF if host readelf is not
called "readelf".

Create app-tools as executable in objdir.

That way they can be judiciously executed from there during
a build.

Add genfs.

genfs is a tool for turning directory hierarchies into an
object file. When the object file in question is baked into
a Rumprun unikernel image, the files from the hierarchies are
(magically) available in the booted unikernel.

app-tools: respect CONFIG_CXX

Use destdir/rumprun-$MACHINE_GNU_ARCH$ as a GNU-like prefix.

Use MACHINE_GNU_ARCH, not MACHINE_ARCH

That matches the terms (and make variables!) with the NetBSD
Makefiles and avoids mismatch lossage.

Install specs to lib instead of share root.

specs are now split into arch and machine specs.

Install brprintmetainfo, it's used by cc

Fixes all kinds of problems in not detecting compile failures.

Problem reported by meena on irc.

Make it rumprun[-MACHINE_ARCH[-PLATFORM]]

That way MACHINE_ARCH (if it exists in the particular name),
is always in the same spot, and the same holds for PLATFORM.

Need separate BRLIBEXECINST and BRLIBEXECDEST

libexecdir is in RRDEST, not INSTALLDIR.

(latter varies depending on if it's a manual "make" or not)

Use more consistent !REPLACE! pattern names.

Build generated tools in object directory.

fixup variable name fsckup

Don't clean non-autogenerated bins.

rumpbake -> rumprun-bake

Puts the tool into the correct namespace (it's a rumprun tool,
not a generic rump kernel tool).

For compat, keep the name "rumpbake" around for now, probably
will go away when we declare things stable.

install toolchain to destdir

Create RUMPRUN_SHCONF, OBJDIR -> RROBJ, DESTDIR -> RRDEST.

CONFIG -> RUMPRUN_MKCONF

If the name of the config file variable is not generic, devs
can set it in the env for convenience. make it so!

Record tool tuple in config.mk and use it.

app-tools Makefile: g/c MACHINE_ARCH assignment

already set in config.mk

g/c unused variable

Give app-tools its own Makefile instead of it being a .inc

This means that app-tools are now built directly by build-rr.sh
instead of being driven by platform/x/Makefile.