docs/manual: Document Kconfig requirements for uboot

When using uboot's legacy build system, the 'make uboot-menuconfig'
and 'make uboot-savedefconfig' targets are not available as they
are created by 'kconfig-package'.

Signed-off-by: Simon Doppler <dopsi@dopsi.ch>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit ca9e55ad11271cb26b0bffba336cf5a0d74ba378)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/make-tips.txt

@@ -35,7 +35,8 @@ some settings in the +.config+ file may hide some targets:
 * +barebox-menuconfig+ and +barebox-savedefconfig+ only work when the
   +barebox+ bootloader is enabled.
 * +uboot-menuconfig+ and +uboot-savedefconfig+ only work when the
-  +U-Boot+ bootloader is enabled.
+  +U-Boot+ bootloader is enabled and the +uboot+ build system is set
+  to +Kconfig+.
 
 .Cleaning: