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/am64x_evm_a53_defconfig b/configs/am64x_evm_a53_defconfig
index 49bfc00..0f54752 100644
--- a/configs/am64x_evm_a53_defconfig
+++ b/configs/am64x_evm_a53_defconfig
@@ -63,6 +63,7 @@
 CONFIG_SPL_DFU=y
 CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_BOOTM_LEN=0x800000
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_DFU=y
 CONFIG_CMD_DM=y