Patch merged into coreboot/master: util/abuild: Fix or disable shellcheck warnings

the following patch was just integrated into master:
commit 02c93b9b1fb2a6fef9c90cc7aef16777e022698b
Author: Martin Roth 
Date:   Wed Nov 30 16:32:20 2016 -0700

util/abuild: Fix or disable shellcheck warnings

This cleans up the shellcheck warnings in abuild.

Warning count:
    1 Unexpected ==.
    1 Use "${var:?}" to ensure this never expands to / .
    1 VARIABLE appears unused. Verify it or export it.
    1 Use "$@" (with quotes) to prevent whitespace problems.
    2 Consider using { cmd1; cmd2; } >> file instead of individual redirects.
    2 Expressions don't expand in single quotes, use double quotes for that.
    3 Prefer [ p ] || [ q ] as [ p -o q ] is not well defined.
    4 $/${} is unnecessary on arithmetic variables.
    5 Check exit code directly with 'if mycmd;', not indirectly with $?.
    5 Use cd ... || exit in case cd fails.
    11 Declare and assign separately to avoid masking return values.
    13 Use $(..) instead of legacy `..`.
    20 Don't use variables in the printf format string.
    104 Double quote to prevent globbing.

Change-Id: I9c77e122435ba87ce3a4aee76b5022f7265f9ef2
    Signed-off-by: Martin Roth 
    Reviewed-on: https://review.coreboot.org/17722
    Tested-by: build bot (Jenkins)
    Reviewed-by: Patrick Georgi

See https://review.coreboot.org/17722 for details.

-gerrit