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/evb-rk3229_defconfig b/configs/evb-rk3229_defconfig
index f44dec0..ebf13c3 100644
--- a/configs/evb-rk3229_defconfig
+++ b/configs/evb-rk3229_defconfig
@@ -36,6 +36,7 @@
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x2000
 CONFIG_SPL_OPTEE_IMAGE=y
 CONFIG_TPL_MAX_SIZE=0x100000
+CONFIG_SYS_BOOTM_LEN=0x4000000
 CONFIG_CMD_GPT=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_USB_MASS_STORAGE=y