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/topic_miamilite_defconfig b/configs/topic_miamilite_defconfig
index 7b97527..97b3f23 100644
--- a/configs/topic_miamilite_defconfig
+++ b/configs/topic_miamilite_defconfig
@@ -38,6 +38,7 @@
 CONFIG_SYS_PROMPT="zynq-uboot> "
 CONFIG_SYS_MAXARGS=32
 CONFIG_SYS_PBSIZE=2077
+CONFIG_SYS_BOOTM_LEN=0x3c00000
 CONFIG_CMD_THOR_DOWNLOAD=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_DFU=y