env: Add support for callbacks to environment vars

Add support for per-variable callbacks to the "hashtable" functions.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

!!!fix comment in callback
diff --git a/common/Makefile b/common/Makefile
index ce30359..04812e9 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -44,6 +44,8 @@
 COBJS-y += cmd_version.o
 
 # environment
+COBJS-y += env_attr.o
+COBJS-y += env_callback.o
 COBJS-y += env_common.o
 COBJS-$(CONFIG_ENV_IS_IN_DATAFLASH) += env_dataflash.o
 COBJS-$(CONFIG_ENV_IS_IN_EEPROM) += env_eeprom.o
@@ -207,6 +209,8 @@
 COBJS-$(CONFIG_ENV_IS_IN_FLASH) += env_flash.o
 COBJS-$(CONFIG_SPL_YMODEM_SUPPORT) += xyzModem.o
 COBJS-$(CONFIG_SPL_NET_SUPPORT) += cmd_nvedit.o
+COBJS-$(CONFIG_SPL_NET_SUPPORT) += env_attr.o
+COBJS-$(CONFIG_SPL_NET_SUPPORT) += env_callback.o
 COBJS-$(CONFIG_SPL_NET_SUPPORT) += env_common.o
 COBJS-$(CONFIG_SPL_NET_SUPPORT) += env_nowhere.o
 COBJS-$(CONFIG_SPL_NET_SUPPORT) += miiphyutil.o