blackfin: Fix build regression due to image size

bf533-stamp, bf538f-ezkit, and cm-bf548 are very space limited.

This was introduced by:
6e0d26c050 (net: Handle ethaddr changes as an env callback)
by enabling CONFIG_REGEX, which is too big for these boards.

This patch disables CONFIG_REGEX at the expense of working with more
than the first ethaddr.

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

configs/bf533-stamp_defconfig

@@ -2,4 +2,5 @@ CONFIG_BLACKFIN=y
 CONFIG_TARGET_BF533_STAMP=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED=y
+# CONFIG_REGEX is not set
 CONFIG_LIB_RAND=y

configs/bf538f-ezkit_defconfig

@@ -2,4 +2,5 @@ CONFIG_BLACKFIN=y
 CONFIG_TARGET_BF538F_EZKIT=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED=y
+# CONFIG_REGEX is not set
 CONFIG_LIB_RAND=y

configs/cm-bf548_defconfig

@@ -2,4 +2,5 @@ CONFIG_BLACKFIN=y
 CONFIG_TARGET_CM_BF548=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED=y
+# CONFIG_REGEX is not set
 CONFIG_LIB_RAND=y