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/orangepi_3_defconfig b/configs/orangepi_3_defconfig
index 824f017..dbca66d 100644
--- a/configs/orangepi_3_defconfig
+++ b/configs/orangepi_3_defconfig
@@ -10,6 +10,7 @@
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL_STACK=0x118000
 CONFIG_SYS_PBSIZE=1024
+CONFIG_SYS_BOOTM_LEN=0x2000000
 CONFIG_PHY_SUN50I_USB3=y
 CONFIG_USB_XHCI_HCD=y
 CONFIG_USB_XHCI_DWC3=y