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/am62x_evm_a53_defconfig b/configs/am62x_evm_a53_defconfig
index 7ebf366..2c0bd4d 100644
--- a/configs/am62x_evm_a53_defconfig
+++ b/configs/am62x_evm_a53_defconfig
@@ -33,6 +33,7 @@
 CONFIG_SPL_DM_MAILBOX=y
 CONFIG_SPL_POWER_DOMAIN=y
 CONFIG_SPL_YMODEM_SUPPORT=y
+CONFIG_SYS_BOOTM_LEN=0x800000
 CONFIG_CMD_MMC=y
 CONFIG_OF_CONTROL=y
 CONFIG_SPL_OF_CONTROL=y