log: Add an implementation of logging

Add the logging header file and implementation with some configuration
options to control it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Makefile b/common/Makefile
index cec506f..f4b6327 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -128,5 +128,6 @@
 obj-$(CONFIG_FSL_DDR_INTERACTIVE) += cli_simple.o cli_readline.o
 obj-$(CONFIG_CMD_DFU) += dfu.o
 obj-y += command.o
+obj-$(CONFIG_$(SPL_)LOG) += log.o
 obj-y += s_record.o
 obj-y += xyzModem.o