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/socrates_defconfig b/configs/socrates_defconfig
index 6215736..60a38ab 100644
--- a/configs/socrates_defconfig
+++ b/configs/socrates_defconfig
@@ -28,6 +28,7 @@
 # CONFIG_BOOTM_PLAN9 is not set
 # CONFIG_BOOTM_RTEMS is not set
 # CONFIG_BOOTM_VXWORKS is not set
+CONFIG_SYS_BOOTM_LEN=0x800000
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_DM=y
 CONFIG_CMD_I2C=y