Allow host programs to be configured

Allow host programs to be configured

Rather than hard-coding in lib/system/system.c, this change adds a set
of #defines for host programs, through the autoheader config.h

These host programs can then be set through configure:

 ./configure HOST_PROG_MOUNT=/usr/bin/mount

Because we need to define nine host programs, we add an autoconf macro
to avoid repeating the definition code.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>