Pass make flags to kernel and lib builds

Pass make flags to kernel and lib builds

Running `make -j4' in /usr/src/distrib/amd64/ramdisk_cd/ et al. executes
make(1) to both build the RAMDISK kernel and build libraries.

Doing so does not propagate the flags specified to the ramdisk_cd
invocation, which in turn means `-j4' for example is ignored and both kernel
and libraries will not be built in parallel.

Pass make(1)'s MFLAGS along to retain relevant flags;  make is clever enough
to separate flags, variable assignments and targets from each other and only
pass along things to `MFLAGS' that'd make sense, i.e. `make -C. -j4 foo=bar'
does *not* pass `-C.' to change directories.
(can be easily tested with `make -p ... | grep MFLAGS'.)

This makes hacking on ramdisks/the installer much faster, espescially since
the `bsd' target does `make clean' and therefore builds a new kernel every
time.

OK deraadt