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/ci20_mmc_defconfig b/configs/ci20_mmc_defconfig
index 983642d..07848a5 100644
--- a/configs/ci20_mmc_defconfig
+++ b/configs/ci20_mmc_defconfig
@@ -34,6 +34,7 @@
 CONFIG_SPL_MMC_TINY=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_BOOTM_LEN=0x4000000
 CONFIG_CMD_DM=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_DHCP=y