runtime: remove paranoia/redundancies

This is all redundant code handled elsewhere by udev and mkinitcpio.
It's also silly to require that the "btrfs" binary be available at
build time, but then also do a runtime check... wat. I suspect this is
leftover from the days when btrfsctl was still around, and the "btrfs"
binary was the new hotness.

Additionally, add a note in the hook help to mention that the edgecase
of not using udev in your initramfs will require the btrfs hook from the
btrfs-progs package.
runtime: defer filesystem type check to mount time

This allows far more freedom in where the hook can be placed in the
HOOKS variable of /etc/mkinitcpio.conf.

This patch also makes the code with comment "must be btrfs or silent
return", actually silent.
runtime: make "kexeced" flag an implicit boolean

This saves us a few bytes on the kernel commandline, which is limited
to ~255 bytes. mkinitcpio will define this in the environment with a
value of "1" if it isn't a key=value pair on the kernel commandline.
runtime: use more reasonable logical constructs

this removes some questionable use of compound commands to use more
readable if/then statements. Generally speaking, the form:

    a && b && c

Is *not* the same as:

    if a; then
      b
      c
    fi

Where the latter form is what's intended, but not what's logically
expressed. While rare, the former case will have unwanted behavior when
the middle of the expression can (and does) fail.
README: remove dangerous advice in install instructions

-Sy is bad juju, and one should not update their sync DBs without doing
a full upgrade, lest they break dependencies. The fallout of doing this
is extremely well documented across the internets.

Additionally, --asroot has been removed from makepkg (this change
manifesting itself in the upcoming pacman 4.2). So, let's not suggest
it.
runtime: simplify kernel version extraction

we can do this all through hexdump, so do that. This also fixes some
potential bugs if the running kernel version contained shell
metacharacters.