Makefile: Rename u-boot-spl.kwb to u-boot-with-spl.kwb

File name with pattern u-boot-spl* is used on all places except in kwb
image for binary with SPL-only code. Combined binary with both SPL and
proper U-Boot in other places has file name pattern u-boot-with-spl*.

Make it consistent also for kwb image and rename u-boot-spl.kwb to
u-boot-with-spl.kwb as this image contains both SPL and proper U-Boot code.

Also update documentation about file name changes.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/Kconfig b/Kconfig
index ef94f88..43d4a48 100644
--- a/Kconfig
+++ b/Kconfig
@@ -456,7 +456,7 @@
 	string "Build target special images"
 	default "u-boot-with-spl.sfp" if TARGET_SOCFPGA_ARRIA10
 	default "u-boot-with-spl.sfp" if TARGET_SOCFPGA_GEN5
-	default "u-boot-spl.kwb" if ARCH_MVEBU && SPL
+	default "u-boot-with-spl.kwb" if ARCH_MVEBU && SPL
 	default "u-boot-elf.srec" if RCAR_GEN3
 	default "u-boot.itb" if !BINMAN && SPL_LOAD_FIT && (ARCH_ROCKCHIP || \
 				ARCH_SUNXI || RISCV || ARCH_ZYNQMP)