Drop CONFIG_ERRNO_STR from SPL

This bloats the code size quite a bit and is less useful in SPL where there
is no command line.

Avoid including this code in SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/Makefile

@@ -23,6 +23,7 @@ obj-$(CONFIG_USB_TTY) += circbuf.o
 obj-y += crc7.o
 obj-y += crc8.o
 obj-y += crc16.o
+obj-$(CONFIG_ERRNO_STR) += errno_str.o
 obj-$(CONFIG_FIT) += fdtdec_common.o
 obj-$(CONFIG_OF_CONTROL) += fdtdec_common.o
 obj-$(CONFIG_OF_CONTROL) += fdtdec.o
@@ -59,7 +60,6 @@ endif
 obj-$(CONFIG_ADDR_MAP) += addr_map.o
 obj-y += hashtable.o
 obj-y += errno.o
-obj-$(CONFIG_ERRNO_STR) += errno_str.o
 obj-y += display_options.o
 obj-$(CONFIG_BCH) += bch.o
 obj-y += crc32.o