initrdscripts: Update install.sh to work with mmc devices

initrdscripts: Update install.sh to work with mmc devices

Fixes [YOCTO #2385]

The installer only searches for hd[ab] sd[ab]. Some newer BSPs have mmcblk
devices that should be used as the install target. These devices also have a
partition prefix (mmcblk0p1 instead of mmcblk01). As they are detected
asynchronously, it is necessary to add the rootwait kernel parameter to avoid
a race condition trying to mount the root device.

As BSPs like the FRI2 and the sys940x have mmc devices and will have a 1.2
release, we should push this to 1.2.1. The changes are perfectly contained and
easily verified.

Test for an mmcblk device and add the p partition prefix if necessary. Add the
rootwait kernel parameter when an mmcblk device is detected.  Replace the series
of explicit umount commands with a single umount using a wildcard. This will
find all the partitions and will not try to unmount non-existant devices. Avoid
copy and paste errors by replacing /dev/${device}${pX} references with the
previously assigned rootfs, bootfs, and swap variables.

These changes have been tested on the FRI2 Sato image which installed to
/dev/mmcblk0 as well as the N450 Sato image which installed to /dev/sda. Both
were successful.

(From OE-Core rev: bf403680d72e360c7382f540ea25cfdcbe77b4e5)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Scott Garman <scott.a.garman@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>