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/uniphier_v7_defconfig b/configs/uniphier_v7_defconfig
index a448d1c..b6b5ca5 100644
--- a/configs/uniphier_v7_defconfig
+++ b/configs/uniphier_v7_defconfig
@@ -27,6 +27,7 @@
 CONFIG_SPL_NOR_SUPPORT=y
 CONFIG_SPL_TARGET="u-boot-with-spl.bin"
 CONFIG_CMD_CONFIG=y
+CONFIG_SYS_BOOTM_LEN=0x2000000
 # CONFIG_CMD_XIMG is not set
 CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS=10
 CONFIG_CMD_GPIO=y