Makefile: Sort u-boot-initial-env output

This will sort the default environment file generated by
"make u-boot-initial-env", but won't sort the default
environment in the compiled u-boot binary. The file
u-boot-initial-env is considered to use for the userpace
environment access tools [1] in case of that the
environments is written the first time into its location.
This is done on the one hand for a better overview and
comparison of the generated environment file. On the other
hand it is to synchronize the output with the script
get_default_env.sh, which generated a sorted default
environment file. The sorting preserves the order of equal
variable names by sorting only the variable name, and
disable the last-resort comparison. After sorting,
unnoticed blank lines at the end move to the top. Avoid
that by removing it before sorting.

[1] https://github.com/sbabic/libubootenv

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Tested-by: Stefano Babic <sbabic@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Marek Behún <marek.behun@nic.cz>
To: u-boot@lists.denx.de
Reviewed-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -2459,7 +2459,8 @@ endif
 
 quiet_cmd_genenv = GENENV  $@
 cmd_genenv = $(OBJCOPY) --dump-section .rodata.default_environment=$@ env/common.o; \
-	sed --in-place -e 's/\x00/\x0A/g' $@
+	sed --in-place -e 's/\x00/\x0A/g' $@; sed --in-place -e '/^\s*$$/d' $@; \
+	sort --field-separator== -k1,1 --stable $@ -o $@
 
 u-boot-initial-env: u-boot.bin
 	$(call if_changed,genenv)