Convert CONFIG_SYS_BOOTM_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_BOOTM_LEN

As part of this, rework error handling in boot/bootm.c so that we pass
the buffer size to handle_decomp_error as CONFIG_SYS_BOOTM_LEN will not
be available to host tools but we do know the size that we passed to
malloc().

Cc: Soeren Moch <smoch@web.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/apple_m1_defconfig b/configs/apple_m1_defconfig
index c4fd803..4ba0933 100644
--- a/configs/apple_m1_defconfig
+++ b/configs/apple_m1_defconfig
@@ -8,6 +8,7 @@
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
+CONFIG_SYS_BOOTM_LEN=0x800000
 # CONFIG_NET is not set
 CONFIG_APPLE_SPI_KEYB=y
 # CONFIG_MMC is not set