Add check_for_file and check_for_tool functions to common.sh

Add check_for_file and check_for_tool functions to common.sh

These functions make it easy to do testing in scripts for required
files and tools and provide useful error messages when the files
or tools are not found.

BUG=chromium-os:11981
TEST=run cros_sign_bootstub in configurations that test all paths in
these functions.

Change-Id: Ia63cfabd1eecba6473331b756486745d71e6b699

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=6f5262d

[davidjames, 2011-03-01 00:09:49]

http://codereview.chromium.org/6591056/diff/1/common.sh
File common.sh (right):

http://codereview.chromium.org/6591056/diff/1/common.sh#newcode660
common.sh:660: error "No ${name} file specified." && exit
1. Looks like this is exiting with a success code. Shouldn't it exit with an
error code? Same comment below.
2. Should we just use 'die' instead if you want to exit?

[robotboy, 2011-03-01 00:20:19]

Too right, fixed.  PTAL.

Thanks,
    Anton

http://codereview.chromium.org/6591056/diff/1/common.sh
File common.sh (right):

http://codereview.chromium.org/6591056/diff/1/common.sh#newcode660
common.sh:660: error "No ${name} file specified." && exit
On 2011/03/01 00:09:49, davidjames wrote:
> 1. Looks like this is exiting with a success code. Shouldn't it exit with an
> error code? Same comment below.
> 2. Should we just use 'die' instead if you want to exit?

Done.

[davidjames, 2011-03-01 00:31:54]

LGTM w/nit

http://codereview.chromium.org/6591056/diff/4001/common.sh
File common.sh (right):

http://codereview.chromium.org/6591056/diff/4001/common.sh#newcode653
common.sh:653: # show the user the name and path of the file that will be used.
Can you show an example use case here? Took me a few minutes to understand what
the padding argument is for, so an example here with the output it generates
would be helpful.