arm64: Drop support for CMDLINE_EXTEND

The documented behaviour for CMDLINE_EXTEND is that the arguments from
the bootloader are appended to the built-in kernel command line. This
also matches the option parsing behaviour for the EFI stub and early ID
register overrides.

Bizarrely, the fdt behaviour is the other way around: appending the
built-in command line to the bootloader arguments, resulting in a
command-line that doesn't necessarily line-up with the parsing order and
definitely doesn't line-up with the documented behaviour.

As it turns out, there is a proposal [1] to replace CMDLINE_EXTEND with
CMDLINE_PREPEND and CMDLINE_APPEND options which should hopefully make
the intended behaviour much clearer. While we wait for those to land,
drop CMDLINE_EXTEND for now as there appears to be little enthusiasm for
changing the current FDT behaviour.

[1] https://lore.kernel.org/lkml/20190319232448.45964-2-danielwa@cisco.com/

Cc: Max Uvarov <muvarov@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Doug Anderson <dianders@chromium.org>
Cc: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/CAL_JsqJX=TCCs7=gg486r9TN4NYscMTCLNfqJF9crskKPq-bTg@mail.gmail.com
Link: https://lore.kernel.org/r/20210303134927.18975-3-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/Kconfig

@@ -1855,12 +1855,6 @@ config CMDLINE_FROM_BOOTLOADER
 	  the boot loader doesn't provide any, the default kernel command
 	  string provided in CMDLINE will be used.
 
-config CMDLINE_EXTEND
-	bool "Extend bootloader kernel arguments"
-	help
-	  The command-line arguments provided by the boot loader will be
-	  appended to the default kernel command string.
-
 config CMDLINE_FORCE
 	bool "Always use the default kernel command string"
 	help

arch/arm64/kernel/idreg-override.c

@@ -188,11 +188,8 @@ static __init void parse_cmdline(void)
 {
 	const u8 *prop = get_bootargs_cmdline();
 
-	if (IS_ENABLED(CONFIG_CMDLINE_EXTEND) ||
+	if (IS_ENABLED(CONFIG_CMDLINE_FORCE) || !prop)
-	    IS_ENABLED(CONFIG_CMDLINE_FORCE) ||
-	    !prop) {
 		__parse_cmdline(CONFIG_CMDLINE, true);
-	}
 
 	if (!IS_ENABLED(CONFIG_CMDLINE_FORCE) && prop)
 		__parse_cmdline(prop, true);