bitbake: usermanual.xml: fix a typo

Fixed:
"should created" -> "should be created"

[YOCTO #5703]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake/lib/bb/build.py: fix the task flags cleandirs

The user manual said:

'cleandirs' - directories which should created before the task runs but should be empty

But it only removes the dir, doesn't create it

[YOCTO #5703]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake: fetch2/git: add nobranch option for SRC_URI to skip SHA validating for branch

For rebased git tree, some commits doesn't exist in any branch, and such commits are
valid in tag, the change is useful for such case.

Signed-off-by: Zhenhua Luo <zhenhua.luo@freescale.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>