Upstreaming addition of shellcheck disables

Summary:
# Description

Upstreaming these shellcheck disable commands.

X-link: facebookexternal/openbmc.arista#361

Test Plan:
Before adding `# shellcheck disable=SC1091`, shellcheck returned the following error:
```
In meta-facebook/meta-meru/recipes-utils/openbmc-utils/files/setup_board.sh line 20:
. /usr/local/bin/openbmc-utils.sh
  ^-----------------------------^ SC1091 (info): Not following: /usr/local/bin/openbmc-utils.sh was not specified as input (see shellcheck -x).
```
Also, `# shellcheck disable=SC2120` is present to avoid confusing the `$2` awk placeholder as a function argument.

Reviewed By: tao-ren

Differential Revision: D56649074

Pulled By: joancaneus

fbshipit-source-id: 6f0f19c93e70a8eb3d7432b459102815bad9b619

meta-facebook/meta-meru/recipes-utils/openbmc-utils/files/board-utils.sh

@@ -177,6 +177,7 @@ bmc_mac_addr() {
     echo "$mac"
 }
 
+# shellcheck disable=SC2120
 userver_mac_addr() {
     # support v4 or v5 eeprom version
     $WEUTIL_CMD scm | grep -E '(Extended|CPU) MAC B' | awk -F': ' '{print $2}'

meta-facebook/meta-meru/recipes-utils/openbmc-utils/files/setup_board.sh

@@ -17,6 +17,7 @@
 # 51 Franklin Street, Fifth Floor,
 # Boston, MA 02110-1301 USA
 
+# shellcheck disable=SC1091
 . /usr/local/bin/openbmc-utils.sh
 
 NETWORK_CONF_FILE="/etc/systemd/network/10-eth0.network"