gawk: Remove References to /usr/local/bin/gawk

Also replace the hashbangs using /bin/gawk to use
${bindir}/gawk

This fixes issues such as
https://github.com/riscv/meta-riscv/issues/384



Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>