Makefile.inc: Stop running git repeatedly for each build

Currently, the coreboot makefiles repeatedly run git to try to set
the KERNELVERSION variable and to fetch the submodules.  This happens
three times for every build.  By exporting a variable, we can catch
this on recursive makes and not run each of these steps again.

Change-Id: I85ab867b40e80c36bd94d48510ffe3252c6cf93f
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/14392
Tested-by: build bot (Jenkins)
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>

Makefile.inc

@@ -22,6 +22,7 @@ CONFIG_FMDFILE:=$(call strip_quotes,$(CONFIG_FMDFILE))
 
 #######################################################################
 # misleadingly named, this is the coreboot version
+ifeq ($(KERNELVERSION),)
 ifeq ($(BUILD_TIMELESS),1)
 export KERNELVERSION := TIMELESS
 else
@@ -31,6 +32,7 @@ export KERNELVERSION := $(strip $(if $(GIT),\
 		$(shell cat $(top)/.coreboot-version),\
 		coreboot-unknown$(KERNELREVISION))))
 endif
+endif
 
 #######################################################################
 # Basic component discovery
@@ -174,6 +176,7 @@ ifeq ($(CONFIG_COVERAGE),y)
 ramstage-c-ccopts += -fprofile-arcs -ftest-coverage
 endif
 
+ifneq ($(UPDATED_SUBMODULES),1)
 # try to fetch non-optional submodules if the source is under git
 forgetthis:=$(if $(GIT),$(shell git submodule update --init))
 ifeq ($(CONFIG_USE_BLOBS),y)
@@ -181,6 +184,8 @@ ifeq ($(CONFIG_USE_BLOBS),y)
 # unless explicitly requested and enabled through --checkout
 forgetthis:=$(if $(GIT),$(shell git submodule update --init --checkout 3rdparty/blobs))
 endif
+export UPDATED_SUBMODULES:=1
+endif
 
 ramstage-c-deps:=$$(OPTION_TABLE_H)
 romstage-c-deps:=$$(OPTION_TABLE_H)