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/mt7628_rfb_defconfig b/configs/mt7628_rfb_defconfig
index 7690213..14fc8b0 100644
--- a/configs/mt7628_rfb_defconfig
+++ b/configs/mt7628_rfb_defconfig
@@ -28,6 +28,7 @@
 CONFIG_SPL_BSS_MAX_SIZE=0x10000
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_NOR_SUPPORT=y
+CONFIG_SYS_BOOTM_LEN=0x1000000
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_XIMG is not set
 # CONFIG_CMD_CRC32 is not set