global: Restrict use of '#include <linux/kconfig.h>'

In general terms, we -include include/linux/kconfig.h and so normal
U-Boot code does not need to also #include it. However, for code which
is shared with userspace we may need to add it so that either our full
config is available or so that macros such as CONFIG_IS_ENABLED() can be
evaluated. In this case make sure that we guard these includes with a
test for USE_HOSTCC so that it clear as to why we're doing this.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>

include/u-boot/ecdsa.h

@@ -8,7 +8,6 @@
 
 #include <errno.h>
 #include <image.h>
-#include <linux/kconfig.h>
 
 /**
  * crypto_algo API impementation for ECDSA;