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/stih410-b2260_defconfig b/configs/stih410-b2260_defconfig
index cb8c73a..c9be056 100644
--- a/configs/stih410-b2260_defconfig
+++ b/configs/stih410-b2260_defconfig
@@ -19,6 +19,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="stih410-b2260 => "
 CONFIG_SYS_PBSIZE=1058
+CONFIG_SYS_BOOTM_LEN=0x1000000
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GPT=y
 CONFIG_CMD_MMC=y