meta-facebook: yosemite4: refactor common function for stability

Refactor `check_nuvoton_board` function to avoid some scritps define
`set -e` in the beginning of `#!/bin/bash` that will think failed when
return value is non-zero and related services might got failed also.

Tested:
Run related services successfully.
root@bmc:/# systemctl status yosemite4-sys-init.service
bmc systemd[1]: Starting Yosemite4 Early System Init...
bmc systemd[1]: yosemite4-sys-init.service: Deactivated successfully.
bmc systemd[1]: Finished Yosemite4 Early System Init.

root@bmc:/# systemctl status mctpd.service
* mctpd.service - MCTP control protocol daemon
    Drop-In: /usr/lib/systemd/system/mctpd.service.d
             `-setup-local-eid.conf, setup-static-endpoints.conf

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Change-Id: I6045d6ae6349cf84f0e920e4d219def602515809
diff --git a/meta-facebook/meta-yosemite4/recipes-yosemite4/plat-svc/files/yosemite4-early-sys-init b/meta-facebook/meta-yosemite4/recipes-yosemite4/plat-svc/files/yosemite4-early-sys-init
index 0ee8a8d..9d10248 100644
--- a/meta-facebook/meta-yosemite4/recipes-yosemite4/plat-svc/files/yosemite4-early-sys-init
+++ b/meta-facebook/meta-yosemite4/recipes-yosemite4/plat-svc/files/yosemite4-early-sys-init
@@ -2,8 +2,7 @@
 # shellcheck source=meta-facebook/meta-yosemite4/recipes-yosemite4/plat-tool/files/yosemite4-common-functions
 source /usr/libexec/yosemite4-common-functions
 
-check_nuvoton_board
-is_nuvoton_board=$?
+is_nuvoton_board="$(check_nuvoton_board)"
 
 enable_all_fan_input()
 {
@@ -58,7 +57,7 @@
 set_gpio NIC2_MAIN_PWR_EN 1
 set_gpio NIC3_MAIN_PWR_EN 1
 
-if [ "$is_nuvoton_board" -ne "1" ]
+if [ -z "$is_nuvoton_board" ]
 then
     # short-term set gpio v0~v3 to input pin for slot5~8 reset button
     mknod -m 660 /dev/mem c 1 1
@@ -69,7 +68,7 @@
 # Enable all fan input as early as possible
 enable_all_fan_input
 
-if [ "$is_nuvoton_board" -eq "1" ]
+if [ -n "$is_nuvoton_board" ]
 then
     # Set interface to SD BIC to I3C for Nuvoton BMC
     set_sd_bic_interface_to_i3c