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/axs103_defconfig b/configs/axs103_defconfig
index 0d17649..29affdc 100644
--- a/configs/axs103_defconfig
+++ b/configs/axs103_defconfig
@@ -21,6 +21,7 @@
 CONFIG_SYS_PROMPT="AXS# "
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=278
+CONFIG_SYS_BOOTM_LEN=0x8000000
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SPI=y