makefile: 'make mrproper' should work even BOARDNAME in .config is wrong

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Makefile: show a error if the defconfig are not found

Signed-off-by: Josh Wu <josh.wu@atmel.com>
conf.c: fix warnning that ignore return value of fgets

fix following warnings:
  conf.c: In function ‘conf_choice’:
  conf.c:323:18: warning: ignoring return value of ‘fgets’, declared with
  attribute warn_unused_result [-Wunused-result]
  conf.c: In function ‘conf_askvalue’:
  conf.c:113:14: warning: ignoring return value of ‘fgets’, declared with
  attribute warn_unused_result [-Wunused-result]

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Makefile: clean the output of making. Use 'make V=1' enable verbose mode

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Makefile: remove duplicated code as we can reuse  mrproper target

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Makefile: use V=1 to enable verbose mode for target: distrib, tarball, tarballx

By default verbose mode is disabled. Which has a cleaner output.
also remove the -v for the command: tar and cpio

Signed-off-by: Josh Wu <josh.wu@atmel.com>