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/emlid_neutis_n5_devboard_defconfig b/configs/emlid_neutis_n5_devboard_defconfig
index d9272ea..73121f2 100644
--- a/configs/emlid_neutis_n5_devboard_defconfig
+++ b/configs/emlid_neutis_n5_devboard_defconfig
@@ -10,4 +10,5 @@
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL_STACK=0x54000
 CONFIG_SYS_PBSIZE=1024
+CONFIG_SYS_BOOTM_LEN=0x2000000
 CONFIG_SUPPORT_EMMC_BOOT=y