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/xenguest_arm64_defconfig b/configs/xenguest_arm64_defconfig
index fec1242..31fb4fd 100644
--- a/configs/xenguest_arm64_defconfig
+++ b/configs/xenguest_arm64_defconfig
@@ -15,6 +15,7 @@
 CONFIG_SYS_PBSIZE=1051
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_BOOTD is not set
+CONFIG_SYS_BOOTM_LEN=0x800000
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_GO is not set
 # CONFIG_CMD_IMI is not set