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/nsim_700be_defconfig b/configs/nsim_700be_defconfig
index 5a09db2..e021cc9 100644
--- a/configs/nsim_700be_defconfig
+++ b/configs/nsim_700be_defconfig
@@ -19,6 +19,7 @@
 CONFIG_SYS_PROMPT="nsim# "
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=279
+CONFIG_SYS_BOOTM_LEN=0x2000000
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_OF_CONTROL=y
 CONFIG_OF_EMBED=y