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/octeontx2_95xx_defconfig b/configs/octeontx2_95xx_defconfig
index 2b3748b..d4f9f56 100644
--- a/configs/octeontx2_95xx_defconfig
+++ b/configs/octeontx2_95xx_defconfig
@@ -37,6 +37,7 @@
 CONFIG_SYS_PROMPT="Marvell> "
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=1050
+CONFIG_SYS_BOOTM_LEN=0x10000000
 # CONFIG_CMD_BOOTEFI_HELLO_COMPILE is not set
 CONFIG_CMD_MD5SUM=y
 CONFIG_MD5SUM_VERIFY=y