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/deneb_defconfig b/configs/deneb_defconfig
index 309545b..ae1179a 100644
--- a/configs/deneb_defconfig
+++ b/configs/deneb_defconfig
@@ -59,6 +59,7 @@
 CONFIG_SYS_PBSIZE=2073
 CONFIG_CMD_CPU=y
 # CONFIG_BOOTM_NETBSD is not set
+CONFIG_SYS_BOOTM_LEN=0x800000
 # CONFIG_CMD_EXPORTENV is not set
 # CONFIG_CMD_IMPORTENV is not set
 # CONFIG_CMD_CRC32 is not set