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/M5208EVBE_defconfig b/configs/M5208EVBE_defconfig
index 4ab888f..858fc5b 100644
--- a/configs/M5208EVBE_defconfig
+++ b/configs/M5208EVBE_defconfig
@@ -16,6 +16,7 @@
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_PROMPT="-> "
 CONFIG_SYS_PBSIZE=276
+CONFIG_SYS_BOOTM_LEN=0x2000000
 CONFIG_CMD_IMLS=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_MII=y