Enable environment variable deletion from obmc-flash-bmc-setenv@.service

This commit extends the functionality of obmc-flash-bmc-setenv@.service,
which calls upon the obmc-flash-bmc script to set an environment
variable.

When working with environment variables in the shell, one can set OR
clear a particular variable using the fw_setenv command. This commit
expands the functionality of the service file to match.

If a name-value pair is specified, this script will set the environment
variable as given. If a name is supplied with no value, the environment
variable with that name will be cleared.

Change-Id: I2eed0148957363668600986f56cb9e678caf855c
Signed-off-by: Michael Tritz <mtritz@us.ibm.com>
diff --git a/meta-phosphor/common/recipes-phosphor/flash/phosphor-software-manager/obmc-flash-bmc b/meta-phosphor/common/recipes-phosphor/flash/phosphor-software-manager/obmc-flash-bmc
index 73f7b51..8d49f50 100644
--- a/meta-phosphor/common/recipes-phosphor/flash/phosphor-software-manager/obmc-flash-bmc
+++ b/meta-phosphor/common/recipes-phosphor/flash/phosphor-software-manager/obmc-flash-bmc
@@ -217,9 +217,13 @@
 
 ubi_setenv() {
     variable=$1
-    varName="${variable%=*}"
-    value="${variable##*=}"
-    fw_setenv "$varName" "$value"
+    if [[ "$variable" == *"="* ]]; then
+        varName="${variable%=*}"
+        value="${variable##*=}"
+        fw_setenv "$varName" "$value"
+    else
+        fw_setenv "$variable"
+    fi
 }
 
 mtd_write() {