busybox: backport dd conv=swab from 1.22

busybox: backport dd conv=swab from 1.22

Some boards have the WLAN EEPROM stored in flash in big-endian format,
whereas the driver requires the EEPROM in little-endian format. The
conv=swab option in dd is particularly useful in this case. This patch
backports this feature from busybox-1.22 based on the following commits:

commit    b941316ae5313be523b64f0a9151ee4decb2b35b dd: support conv=swab
commit    8395bd3f52f8ed46fa3ffc316b2d113afa748bae dd: fix example in a
comment. No code changes.
commit    5b9910f0a4a1b7976c46e6f849aaa263180e5521 dd: fail if swab is
attempted on odd-sized block
commit    0ff0b320a2dbb5e0b5fa245ffd9b2648d7026843 dd: code shrink

v2: include the commit messages from upstream

Signed-off-by: Matti Laakso <malaakso@elisanet.fi>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40327 3c298f89-4303-0410-b956-a3cf2f4a3e73